whatsapp registration into a group

On this page: [login to view URL] there is a registration form which register a user into a email provider group. The form works fine. I want that by pressing 'send' in the form, the form will continue to function ok (the DB will update with the new user's details) but the user will activate the whatsapp link to join a group: [login to view URL] (correct link will provided) The whatsapp application will start upon pressing 'send' and the new user will enter automatically into the group on his cellular phone.