CategoriesController.cs

[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Create([Bind("Id,Name,UserId")] Category category)
{
	// Populate UserId select list.
	ViewData["UserId"] = new SelectList(_context.Users, "Id", "Id", category.UserId);

	// If ModelState isn't valid, return to view immediately
	if (!ModelState.IsValid)
	{
		return View(category);
	}

	// Add category, save changes and redirect to Category/Index - Categories List page
	_context.Add(category);
	await _context.SaveChangesAsync();
	return RedirectToAction(nameof(Index));
}