currently it is not possbile to inline mentions in a message that's postet into a channel via Flow into Teams.
There are several thread posts, but I did not find an Idea yet.
So because this would be a killer feature that also others need, please support a syntax for text and in the rich editor like:
Hello @TEAMSUSERNAME (this is a teams post)
It would also help, if the mention could be done by email adress or Full Name (eg @'Joen Doe').