1. Create a RequiredIfAttribute class:

        public class RequiredIfAttribute : ValidationAttribute
        {
            private readonly string _comparisonProperty; 
    
            public RequiredIfAttribute(string comparisonProperty)
            {
                _comparisonProperty = comparisonProperty;
            }
    
            protected override ValidationResult IsValid(object value, ValidationContext validationContext)
            {
                ErrorMessage = ErrorMessageString;
    
                //get entered WorkPass Type
                var workpasstype = value == null ? "" : value.ToString();
                var property = validationContext.ObjectType.GetProperty(_comparisonProperty);
                if (property == null)
                    throw new ArgumentException("Property with this name not found");
                //get the about value
                var workpasstypeValue = (string)property.GetValue(validationContext.ObjectInstance);
    
                if (workpasstype == "" && workpasstypeValue != null)
                    return new ValidationResult(ErrorMessage);
    
                return ValidationResult.Success;
            }
        }

 2. Create a BiodataViewModel model contains the relate properties:

        public class BiodataViewModel
        {
            //--WorkpassType ---//
            [Required(ErrorMessage = "Please Select WorkPass Type")]
            [Display(Name = "WorkPass Type"), StringLength(20)]
            public string WorkpassType { get; set; }

            //--Workpass Expiry Date ---//
            //[Required(ErrorMessage = "Please Select WorkPass Expiry Date")]
            [Display(Name = "WorkPass Expiry Date")]
            [DisplayFormat(DataFormatString = "{0:dd/MM/yyyy}", ApplyFormatInEditMode = true)]
            [DataType(DataType.Date)]
            [RequiredIf("WorkpassType", ErrorMessage = "Please select WorkPass Expiry Date")]
            public DateTime? WorkpassExpiryDate { get; set; }
        }

 3. Create a _BioData.cshtml page like this:

        @model Core3Razor.Data.BiodataViewModel

        <div asp-validation-summary="ModelOnly" class="text-danger"></div>
        <div class="form-group">
            <label asp-for="WorkpassType" class="control-label"></label>
            <input asp-for="WorkpassType" class="form-control" />        
            <span asp-validation-for="WorkpassType" class="text-danger"></span>
        </div> 
        <div class="form-group">
            <label asp-for="WorkpassExpiryDate" class="control-label"></label>
            <input asp-for="WorkpassExpiryDate" class="form-control" />
            <span asp-validation-for="WorkpassExpiryDate" class="text-danger"></span>
        </div> 

 4. Create a CreateBioData page with page model 

  the CreateBioData.cshtml

        @page
        @model Core3Razor.Pages.CreateBioDataModel 
        <div class="row">
            <div class="col-md-4"> 
                <form method="post" id="FormBiodata" enctype="multipart/form-data">
                    <partial name="_BioData" for="Biodata" /> 
                    <div class="form-group row">
                        <div class="col-sm-10" style="margin-left:800px;">
                            <input type='submit' id="btnsave" class='btn btn-success' value='Save' />
                            <input type="submit" id="BtnSubmit" asp-page-handler="InsertBioData" style="display:none;" />
                        </div>
                    </div>
                </form>
            </div>
        </div>

        @section Scripts{
            @{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
            <script type="text/javascript">
            $(function(){ 
                    //client side validation
                    $('#btnsave').click(function (e) {
                        e.preventDefault();           
                        var WorkPassType = $('#Biodata_WorkpassType').val();

                        if(WorkPassType != ""){
                            var ExpiryDate = $('#Biodata_WorkpassExpiryDate').val();
                            if(ExpiryDate == ""){
                                $('#Biodata_WorkpassExpiryDate').next("span").html("Please Enter WorkPass Expiry Date")
                            }
                            else{
                                $('#Biodata_WorkpassExpiryDate').next("span").html("");
                                $('#FormBiodata').submit();
                            }
                        } 
                        else{
                             $('#FormBiodata').valid();
                        }
                    });
            });
            </script>
        }

   The CreateBioData.cshtml.cs 

        public class CreateBioDataModel : PageModel
        {
            [BindProperty]
            public BiodataViewModel Biodata { get; set; }

            public void OnGet()
            {
                Biodata = new BiodataViewModel();
            }

            public IActionResult OnPost()
            {
                //server side validation
                if (!ModelState.IsValid)
                {
                    return Page();
                }
                var WType = Biodata.WorkpassType;
                var WDate = Biodata.WorkpassExpiryDate;
                return RedirectToPage(nameof(Index));
            }

            public void OnPostInsertBioData()
            {
                var newitem = Biodata;
            }
        }