1. RequiredIfAttribute.cs

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

namespace Core3Razor.ModelValidation
{
    public class RequiredIfAttribute : ValidationAttribute, IClientModelValidator
    {
        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 workpasstype value
            var workpasstypeValue = (string)property.GetValue(validationContext.ObjectInstance);

            if (workpasstype == "" && workpasstypeValue != null)
                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 select the WorkPass type, please select WorkPass Expiry Date");              
        }
        private static bool MergeAttribute(IDictionary<string, string> attributes, string key, string value)
        {
            if (attributes.ContainsKey(key))
            {
                return false;
            }

            attributes.Add(key, value);
            return true;
        }
    }
}

2. BiodataViewModel.cs 

using Core3Razor.ModelValidation;
using System;
using System.ComponentModel.DataAnnotations;

namespace Core3Razor.Data
{
    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")]
        [RequiredIf("WorkpassType", ErrorMessage = "Please select WorkPass Expiry Date")]
        [Display(Name = "WorkPass Expiry Date")]
        [DisplayFormat(DataFormatString = "{0:dd/MM/yyyy}", ApplyFormatInEditMode = true)]
        [DataType(DataType.Date)]
        public DateTime? WorkpassExpiryDate { get; set; }
    }
}

3. _BioData.cshtml

@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. 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">       
        $.validator.addMethod('requiredif', function (value, element, params) {
            //get the entered workpassType
            var type = $("#Biodata_WorkpassType").val();
            //get the workpassExpiryDate
            var workpassExpiryDate = value;
            // if the ype is not empty and the workpassExpiryDate is "", validate fail return false and show the error message.
            if(type != "" && workpassExpiryDate == "")
            {
                return false;
            } 
            return true;
        });
        $.validator.unobtrusive.adapters.add('requiredif', function (options) {
            var element = $(options.form).find('#Biodata_WorkpassType');
            options.rules['requiredif'] = [element];
            options.messages['requiredif'] = options.message;
        });
    //$(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>
}

5. CreateBioData.cshtml.cs

using Core3Razor.Data;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;
using System;

namespace Core3Razor.Pages
{
    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;
        }
    }
}