How do i convert a user input value to a SOUNDEX value? using .net core webAPI 7

Question

I want the user submitted value to arrive in the MS SQL DB as a SOUNDEX value. eg. soundex(Amnesia) = A520

I have this Function in my dbcontext:

[DbFunction(name: "SOUNDEX", IsBuiltIn = true, IsNullable = false)]
     public string   Soundex(string query   )
     {
         throw new NotImplementedException();
     }

This works fine if I perform a GET but I want to POST.

this is the NameRequest DTO

name = {
Sndx: '',
FirstName: 'Biggus',
LastName: 'Dikuss'
}

In my controller I have:-

public async Task<ActionResult<NameRequest>> PostName(NameRequest name)
    {
        name.Sndx = _context.Soundex(name.FirstName); //it fails here
        await _context.SaveChangesAsync();
        return Ok();
    }