1. UserProfile.cs:

        using MVCWebApplication.CustomValidation;
        using System.ComponentModel.DataAnnotations;
        using System.ComponentModel.DataAnnotations.Schema;

        namespace MVCWebApplication.Models
        {
            public class UserProfile
            {
                public int UserID { get; set; }

                [Required]
                public string Name { get; set; }

                [NotMapped]
                public bool IsChangePassword { get; set; }
         
                [DataType(DataType.Password)]
                [Display(Name = "New Password")]
                [RequiredIf("IsChangePassword")]
                [StringLength(250, MinimumLength = 6, ErrorMessage = "Minimum 6 character password required")]
                public string? newPassword { get; set; }
                [NotMapped]
                //[Required(ErrorMessage = "Verify Password required")]
                [CompareAttribute("newPassword", ErrorMessage = "Password doesn't match.")]
                [DataType(DataType.Password)]
                public string? verifyPassword { get; set; }
            }
        }

2. RequiredIfAttribute.cs: 

        using Microsoft.AspNetCore.Mvc.ModelBinding.Validation;
        using System.ComponentModel.DataAnnotations;

        namespace MVCWebApplication.CustomValidation
        {
            [AttributeUsage(AttributeTargets.Property)]
            public class RequiredIfAttribute : ValidationAttribute, IClientModelValidator
            {
                private string PropertyName { get; set; } 
                public RequiredIfAttribute(string propertyName)
                {
                    PropertyName = propertyName;
                } 
                protected override ValidationResult IsValid(object value, ValidationContext validationContext)
                {
                    //get the userprofile instance
                    object instance = validationContext.ObjectInstance;
                    Type type = instance.GetType();
                    //get the property value based on the property name. Here we can get the IsChangePassword value.
                    bool.TryParse(type.GetProperty(PropertyName).GetValue(instance)?.ToString(), out bool propertyValue);

                    //check whether the IsChangePassword checkbox is checked, if yes, validate the password. if not, ignore the password validation.
                    if (propertyValue && string.IsNullOrWhiteSpace(value?.ToString()))
                    {
                        return new ValidationResult(ErrorMessage);
                    } 
                    return ValidationResult.Success;
                } 
                public void AddValidation(ClientModelValidationContext context)
                {
                    MergeAttribute(context.Attributes, "data-val", "true");
                    MergeAttribute(context.Attributes, "data-val-requiredif", "Since checked the ChangePasswork, please enter tne new password");
                }
                public static bool MergeAttribute(IDictionary<string,string> attributes, string key, string value)
                {
                    if (attributes.ContainsKey(key))
                    {
                        return false;
                    }
                    attributes.Add(key, value);
                    return true;
                }
            }
        }

3. Controller:


        public IActionResult EditUser()
        {
            var user = new UserProfile()
            {
                UserID = 101,
                Name = "Tom",
                IsChangePassword = false,
            };
            return View(user);
        }
        [HttpPost]
        public IActionResult EditUser(UserProfile user)
        {
            if (ModelState.IsValid)
            {
                //update user

                return View(nameof(Index));
            }
            return View(user);
        }

4. EditUser.cshtml page:

        @model MVCWebApplication.Models.UserProfile
 
        <div class="row">
            <div class="col-md-4">
                <form asp-action="EditUser">
                    <div asp-validation-summary="ModelOnly" class="text-danger"></div>
                    <div class="form-group">
                        <label asp-for="UserID" class="control-label"></label>
                        <input asp-for="UserID" class="form-control" />
                        <span asp-validation-for="UserID" class="text-danger"></span>
                    </div>
                    <div class="form-group">
                        <label asp-for="Name" class="control-label"></label>
                        <input asp-for="Name" class="form-control" />
                        <span asp-validation-for="Name" class="text-danger"></span>
                    </div>
                    <div class="form-group">
                        <label class="form-check-label">
                           @Html.DisplayNameFor(model => model.IsChangePassword) 
                        </label>
                        <input class="form-check-input" type="checkbox" asp-for="IsChangePassword" checked="@Model.IsChangePassword" /> 
                    </div>
                    <div class="form-group">
                        <label asp-for="newPassword" class="control-label"></label>
                        <input asp-for="newPassword" class="form-control" />
                        <span asp-validation-for="newPassword" class="text-danger"></span>
                    </div>
                    <div class="form-group">
                        <label asp-for="verifyPassword" class="control-label"></label>
                        <input asp-for="verifyPassword" class="form-control" />
                        <span asp-validation-for="verifyPassword" class="text-danger"></span>
                    </div>
                    <div class="form-group">
                        <input type="submit" value="Save" class="btn btn-primary" />
                    </div>
                </form>
            </div>
        </div>
  
        @section Scripts{
            @{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
            <script type="text/javascript">       
                $.validator.addMethod('requiredif', function (value, element, params) {
                    //get the IsChangePassword value.
                    var ischangepassword = $("#IsChangePassword").is(":checked");
                    //get the new password
                    var newpassword = value;
                    // if the ype is not empty and the workpassExpiryDate is "", validate fail return false and show the error message.
                    if(ischangepassword && newpassword == "")
                    {
                        return false;
                    } 
                    return true;
                });
                $.validator.unobtrusive.adapters.add('requiredif', function (options) {
                    var element = $(options.form).find('#IsChangePassword');
                    options.rules['requiredif'] = [element];
                    options.messages['requiredif'] = options.message;
                }); 
            </script>
        }