As Guriyot Singh wrote,
the "banner" has not been very detailed..... however the question has got a +1 !
So I try to guess
It's this one !
In case it is , it's not a plugin. It belongs to the core of q2a, and you can enable it in the Admin panel :
Admin ---> Users ---> Notice at top for first time visitors
Admin ---> Users ---> Notice at top for new registered users
There you can insert your custom message, using HTML too, for the 2 different types of notice.
I hope I've guessed !