$('#btnVolunteerSaveBtn').on('click', function() // on click of save button

 {
        if (document.getElementById('RadioNo').checked) { //ID of radio button NO
            var checking = $('#Donation').val(); //ID of textbox from where the value is to be taken if RadioButton No is selected
            if (checking == "") {
                //if nothing is entered, stop from saving in DB
            }
            else
            {
                x = $('#Donation').val(); //ID of textbox from where the value is to be taken if RadioButton No is selected
                $.ajax({
                    url: '@Url.Action("DonationValue","VolunteerInfo")',
                    data: { name: x },
                    type: "POST"
                });
            }
        }
        else
        {
            x = $('#GetNames').val(); //ID of textbox from where the value is to be taken if RadioButton Yes is selected
            $.ajax({
                url: '@Url.Action("DonationValue","VolunteerInfo")',
                data: { name: x },
                type: "POST"
            });
        }

    });

[HttpPost]
        public ActionResult AddVolunteer(VolunteerInfo viewModel)
        {
            if (!ModelState.IsValid)
            {
                return View("AddVolunteer", viewModel);
            }
        var volunteer = new VolunteerInfo()
        {
            Name = viewModel.Name,
            BirthdayDateTime = viewModel.BirthdayDateTime,
            DonationForWhom = DonationValue() //error here. Expecting a parameter
        };

        _context.VolunteerInfos.Add(volunteer);
        _context.SaveChanges();
        return RedirectToAction("Index", "Home");

    }

    public string DonationValue(string name)
    {
        return name; //Trying to pass this value above
    }

public string DonationValue(string name = null)
        {
            return name; //Trying to pass this value above
        }