StartupPage.cs

public abstract class StartupPage : ContentPage, IStartupTask
{
   ...
    async Task<IStartupTaskParameters> IStartupTask.RunAsync(IStartupTaskParameters parameters)
    {
        _tcs = new TaskCompletionSource<IStartupTaskParameters>();

        await OnStartAsync(parameters);

        await App.Current.MainPage.Navigation.PushModalAsync(this);

        return await _tcs.Task;
    }

    private TaskCompletionSource<IStartupTaskParameters> _tcs;
}