If you try to post just to have the 10 post count, I can guarantee you that you will not be validated for the airdrop.
It must be rich posts.
Some people spam the last days just to reach 10 per week. It won't make them join if they do that.

And to answer your question, you can post wherever you want on the bitcointalk forum. What matters is that you show some interest in the discussions, and add value.

Good luck