How to post data into a foreign Key table

Question

Please i need help, i have two tables User and ExistingUser

 

i have a foreign key UserId in the ExistingUser table,

 

And on the user interface i can search for the names from the User Table,

 

how can i post the name into the UserId(FK)column in the ExistingUser table.

 

i'm using asp.net core 2.0 mvc

[HttpPost]
public async Task<IActionResult> Existing(BigView big)
{
if (!ModelState.IsValid)
{
return View();
}
var student = (from p in _context.Users where p.UserId== p.UserId select p).First();
_context.ExistingUser.Add(big.ExistingUser);
await _context.SaveChangesAsync();
return RedirectToAction("Existing");
}