Pre-approves an inquiry on the host's behalf, which tells the guest they may book. On Airbnb this is the same as pressing Pre-approve. On Booking.com it approves the guest's request to book. On VRBO it sends the inquiry's quote back unchanged, as the VRBO app does, with the message you send to the guest (required on VRBO). VRBO support is built from a captured VRBO app request and is not yet tried on a live account; the VRBO app also sends an App Check header Host Tools can't send, so VRBO may refuse it.
The reservation must have source "Airbnb", "Booking" or "VRBO" and status "inquiry". Requires the reservations:write scope.

The channel is the source of truth: Host Tools sends the pre-approval to the channel, then refreshes the reservation from it. The new status (preapproved) shows up in /api/getReservation/{reservationID}, and the reservation webhook fires, shortly after the call returns, not in the response itself.