public class UsersAuthenticateCommand : IRequest<IActionResult>
{
    [Required]
    public string Username { get; set; }

    [Required]
    public string Password { get; set; }
}

public class UsersAuthenticateCommandHandler : IRequestHandler<UsersAuthenticateCommand, IActionResult>
{
    private IUserService _userService;

    public UsersAuthenticateCommandHandler(IUserService userService)
    {
        _userService = userService;
    }

    public async Task<IActionResult> Handle(UsersAuthenticateCommand command, CancellationToken cancellationToken)
    {
        UsersAuthenticateCommandResult response = _userService.Authenticate(command);

        if (response == null)
            return new BadRequestObjectResult(new { result = "Username or password is incorrect" });

        return new OkObjectResult(response);
    }
}