AddUserToRole

[HttpPost("User/{userEmail}/Role")]
public async Task<IActionResult> AddUserToRole(string userEmail, [FromBody] string roleName)
{
    var user = _userManager.Users.SingleOrDefault(u => u.UserName == userEmail);

    var result = await _userManager.AddToRoleAsync(user, roleName);

    if (result.Succeeded)
    {
        return Ok();
    }

    return Problem(result.Errors.First().Description, null, 500);
}