The "Events / Challenges" cagetory is for community events and creative challenges within the community, so I think it fits. You can check out what others have posted for contests / challenges.
I never ran an art competition, but joined in a few, so here are some observations.
Platforms / How to join? 
You can possibly share your art competition by: Tapas forum, bonus page on your comic, Instagram, etc. Keeping instructions simple is good such as "Draw this in your style (DTIYS)" or "Draw a Cookie Monster". Bullet-point lists may be easier to read than paragraphs.
Think about how people should submit their contest entry to you. For Tapas forum, they can reply with a drawing on the same post. For other social media platforms like Instagram, they can tag @you or tag "A_very_specific_thing". Direct messages (DM) may work too. IF there are mutiple ways to submit, just let people know b/c everyone haunts different social media. However, more social media is more work, so maybe choose like 1-3 ways for people to submit their contest entry.
Are mutiple submissions allowed? -- a.k.a. 2 drawings from the same person. This might be worth noting if your art challenge is more imagination-based where you can get very different drawings from the same challenge. For example, there was a contest to draw characters in possible futures -- I drew one as an inventor apprentice in fairytale times, and the other one scavenging in a zombie apocalypse; for a DTIYS - it is typically one drawing.
Good luck with your art competition! 
