Opens the booking form for one offer from a comparison, identified by its ref; the form renders below the message. Read-only: the call submits, stores and sends nothing, and takes no personal data. In the form the user enters the contract details (page 1), then the IBAN and consent (page 2), and files the request there. Messages such as 'Bitte öffne das Buchungsformular', 'ich möchte diesen Tarif', 'den nehme ich', 'book #2' or 'switch me to this', usually sent by the card's 'Jetzt buchen' button, ask for this form. language sets the language of the form. There is no self-service checkout link. Keywords: Buchungsformular, Tarif buchen, wechseln, Wechsel beauftragen, Vertrag abschließen, open booking form, book, switch, sign up.