Post flair is not (yet) in lemmy. It is a feature that is supposed to be in lemmy 1.0 and we have tried to make PieFed's post flair compatible with what lemmy's flair will eventually look like (it was just a pull request at the time though, so it might need some tweaking as 1.0 gets closer).
There are some lemmy frontends that will convert some text in the title into something that visually looks like post flair. Photon is one that I know does this. So, if your post title is [Premiere] Awesome show debuts, then photon will instead render it as a post with the title Awesome show debuts and then tack on a Premiere post flair.