Originally Posted by exploreaswego

Not sure if this helps, but I called the Diamond Desk via the HHonors app to redeem my FN cert.
Punched in my Diamond Member # on the automated line, was immediately connected to a local rep.
Everything done is less than 5 minutes, super pleasant experience.

Go to the app > Tap Contact on the bottom right corner -> Tap Talk to a Customer Care agent -> The number for diamond desk should show up.
Mine shows:
855-292-557 (For US toll free)
800-4445-8667 (International UIFN)

Hope this helps!