Yep, you can do that exactly! We need to write some support articles to walk through the steps, but that's what we recommend as a standard process.
The contact info form on the renter agreement signing link will detect and blank out any of the @airbnb.com or @homeaway.com proxy email addresses so that the guest can enter a real email address. Once they do, that real email address will go on file. It doesn't remove the proxy email address right now, but we should add an option for that.
For Airbnb, you can set up a create trigger that'll go out automatically as soon as the calendar import picks up the booking because they include the email address there. Here's an example: https://www.ownerrez.com/support/articles/airbnb-request-for-contact-info-real-email-address-and-signed-renter-agreement
For VRBO, they don't include an email address in the iCal, but our reservations and channel bridge imports have an option to send create triggers when populating the email -- those imports do have email addresses, so you can set up a create trigger for VRBO that will be sent as soon as you do import the email address.
