My mastery of the dark arts is truly unparalleled 😎
Actual answer
The option doesn't appear on lemmy-ui for reasons beyond me, but it's not blocked in the API so I just did this with curl:
cu -H 'Content-Type: application/json' https://feddit.uk/api/v3/community/mod -X POST -d '{"community_id": 502135, "person_id": 17930, "added": true}'