Collecting hotels.com rewards points for a third-party booking
Upvote:3
I found my own answer shortly after posting. Although it's not immediately obvious how to do it, instructions are available from hotels.com customer service:
Sign in to your Hotels.com account, go to Find bookings, fill out the form and Submit.
Select View your reservation.
Scroll to the Hotels.com™ Rewards box and select Sign in and attach this booking to your account. If you don't see this link, your booking is already attached to your account.
You should then see this message: "Success! You have successfully added this booking to your Hotels.com™ Rewards account."