I think this task is too light to be categorized as a bounty task (cmiiw), not heavier than the task of subscribing to social media. Check forum rules:
Incentivizing posting within one or several threads via
low effort tasks (e.g.
signups or proof of participation for liking, following, subscribing, retweeting, tweeting a single tweet, joining a channel or group, etc.) is not allowed as it falls under the "no altcoin giveaways" rule (
https://bitcointalk.org/index.php?topic=2103687.0):
Fyi, the penalty is a temporary ban. I saw some who got it appealed in the Meta section.
I suggest you to use google form for joining method and use some
google form features to prevent or minimize cheating participants, and enforce minimal ranking requirements.