<form method="post" action="Login">
    <table class="tableLogin">
        <tr class="tableLoginText" ><td><input class="input" type="text" name="Username" placeholder="Имя пользователя" /> </td></tr>
        <tr class="tablePasswordText"><td> <input class="input" type="password" name="Password" placeholder="Пароль" /> </td></tr>
        <tr class="tableButtonLogin"><td>  <input class="buttonLgn" type="submit" name="ButtonLogin" value="ВОЙТИ" /> </td></tr>
    </table>
</form>

[HttpPost]
[Route("Login")]
public string Login(LoginUser user)
{
    if (user.Username == null || user.Password == null)
    {
        return "<h1>Неверные значения логина или пароля</h1>";
    }
    return user.Username + user.Password;
}