Once the challenge end, the admin will validate all the entries and a link to claim the reward will be posted in the thread of the challenge.
Usually it will be available to claim within 24-48hours after the challenge end, the more entries = the longer time for them to validate IMO.
Did you manage to hit good multiplier in the current chosen slot "Fire Portals"?
I have submitted my entry, few post after @Betwrong entry and I'm sure I'll be one of the top 20