ok, thank you

That is typically only available in [i]problem/issue[/i] oriented forums like [i]need help[/i].
There is a section called "post an icon" just above the subject line where you'll find a [u]green check mark[/u] to tick.

You can also edit the subject line at the beginning or end to say something like "solved".

Hi,

I cannot find a possibility to mark a topic with "finished" or similar to show other users that there is a satisfying answer.

:-)

Frank