I totally agree, there is no need for any key in this app. Looks like a scam attempt.
They say "It just for sure that you are a steem user." but why would they care about that ? If the user is not a Steem user then too bad for him. Honestly, just specifying a Steem username is enough, there is no need for anything else. I wouldn't care if someone linked its revenue to my account, that would be free money after all !
Anyway, I agree with you that it's extremely fishy. I'm flagging it for being most probably a scam.
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