You cannot require participants to post "proof of authentication"
As an alternative, you can have participants PM you this "proof"
But most of the bounties and airdrops require to post proof of authentication.
In games and rounds, users are required to post their username too.
Moreover, if I don't require them to post a proof, they will definitely cheat with other people's username.
If you are giving away altcoins it is an altcoin giveaway.
If you require participants to complete some nontrivial, non-low effort task, you can have participants post a report, this is what you refer to ‘proof of authentication’.
I don’t think downloading a program and filling out a form meets the above criteria. You cannot give incentives for people to post low effort posts in your thread.
Like I said, you can have participants send a PM to prove that they are who they say they are.