So you have a form with a normal complete your details and a SignUp button that runs another script on submit, then you have your Anchor button for facebook which is fine, all you need to do is set the link to what that button must do.
Click the button in design view and then as per the Wappler Docs
This link takes you right to the correct step that shows you how to add your link.
If I were you though I would review and follow this Wappler document as close as possible.