Pass data between Page and Activity for external application

Ciao.
I've a PCL Xamarin.Forms application.
I've to call an external application, so I have to create a PageRenderer and an Activity.
In the activity, using StartActivityForResult, I can start the external application.
I've to pass some data to the external application. So, I've to pass data from the Page (in my PCL Xamarin.Forms app) and the activity.
How can I do this?
I've take a look to "Forms2Native" example, but It doesn't pass data ...
Thanks
Alessandro