@Ronald880 Good Morning! You will have to make sure that the signing of a rental agreement is stated in your house rules as well as things to know so the guest is aware of this.
You can then set an automated quick reply in Airbnb messaging to fire once the booking comes in asking them for their email address so you can send them the rental agreement for signing. I am not positive but you may be able to also include a link to the rental agreement in the quick reply. I beleive you can add a link to a guest guide so it may be possible to add rental agreement link.
Karen