Unit testing an endpoint that executes a command

Program.cs

sealed class MyRequest
{
    public int Id { get; set; }
}

public sealed class MyCommand : ICommand<int>
{
    public string Identity { get; set; }
}

sealed class MyCommandHandler : ICommandHandler<MyCommand, int>
{
    public Task<int> ExecuteAsync(MyCommand command, CancellationToken ct)
        => Task.FromResult(int.Parse(command.Identity));
}

sealed class MyEndpoint : Endpoint<MyRequest, int>
{
    public override void Configure()
    {
        Post("test");
        AllowAnonymous();
    }

    public override async Task HandleAsync(MyRequest r, CancellationToken c)
    {
        var command = new MyCommand { Identity = r.Id.ToString() };
        var result = await command.ExecuteAsync();
        await SendAsync(result);
    }
}

UnitTest.cs

[Fact]
public async Task Endpoint_With_Command_Inside()
{
    //arrange
    var ep = Factory.Create<MyEndpoint>();
    var req = new MyRequest { Id = 123 };

    var fakeHandler = A.Fake<ICommandHandler<MyCommand, int>>();
    A.CallTo(() => fakeHandler.ExecuteAsync(A<MyCommand>.Ignored, A<CancellationToken>.Ignored))
     .Returns(Task.FromResult(123));
    fakeHandler.RegisterForTesting(); // this is the important bit

    //act
    await ep.HandleAsync(req, default);

    //assert
    ep.Response.Should().Be(123);
}