StartupPage.cs

public abstract class StartupPage : ContentPage, IStartupTask
{
    protected virtual Task<bool> CanRunAsync() => Task.FromResult(true);

    protected async Task CompleteAsync()
    {
        await Navigation.PopModalAsync(false);

        _tcs?.SetResult(true);

    }

    Task<bool> IStartupTask.CanRunAsync() => CanRunAsync();

    async Task IStartupTask.RunAsync()
    {
        _tcs = new TaskCompletionSource<bool>();

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

        await _tcs.Task;
    }

    private TaskCompletionSource<bool> _tcs;
}