[nukme]

Originally Posted by Rainrider ChallengeModeCompleteBannerMixin is only available after Blizzard_ChallengesUI has been loaded. So either load the addon yourself prior to hooking, or listen to ADDON_LOADED and do the hook there.

I have finished my code. Turns out the frame name should be ChallengeModeCompleteBanner, as defined in the .xml file.

So the right hook is as below, when Blizzard_ChallengesUI is loaded.

hooksecurefunc(ChallengeModeCompleteBanner,"PlayBanner",Screenshot)

And one can change the hook a bit to allow for some delay to wait for the whole complete banner.

However I don't fully understand the structure of blizz code, as the relationship between ChallengeModeCompleteBanner and ChallengeModeCompleteBannerMixin confuses me, and I have to do some trial and error to comfirm the right hook combination.