Found on a private fb group.
Go to Qatar Airways. (I used mobile browser, and was not logged in, but have replicated on desktop too).
Click on Experiences, then Dining.
Scroll down and choose one of the dining experiences.
On the right you’ll see a ticket icon. Tap that. Receive personalised discount code. I got offered 10% off, I’ve seen others claim they got 20%. Might depend what experience you select first, maybe?