curl -sS -X POST 'https://matrix.example.com/_synapse/admin/v1/rooms/!abc123:example.com/make_room_admin' -H "Authorization: Bearer $ACCESS_TOKEN" -d '{"user_id":"@alice:example.com"}'

Grants the user the highest power level available to a local member of the room. If they are not in the room and it is publicly joinable, they are invited automatically. Leave the body empty to promote the calling admin instead. Useful for handing a room over to someone else.