Xamarin forms UITest

I'm trying use app.Invoke in a XF UITest app.
I've followed the instructions from here: https://developer.xamarin.com/guides/testcloud/uitest/working-with/backdoors/
In my AppDelegate I have the
[Export("methodName:")]
public NSString MethodName(NSString arg){
return new NSString("method");
}

And in the UITest class I have:
[Test]
public void AppLaunches ()
{
app.Invoke ("methodName","val");
}

Error while performing Invoke("methodName", "val")
Exception: System.Exception: Invocation failed: The backdoor: 'methodName' is undefined
at Xamarin.UITest.iOS.iOSApp+c__AnonStorey27.<>m__0 () <0x4861b40 + 0x00137> in :0
at Xamarin.UITest.Utils.ErrorReporting.With[T] (System.Func`1 func, System.Object[] args, System.String memberName) <0x48617e8 + 0x00034> in :0

Any help?

Thanks!