Good job tagging this as levitation, too. You posted this from your Fedia account, so the tag works! Edit: Actually, this came from your Lemmy account, but the tag still works somehow. Did Mbin allow non-Mbin users to add tags that work?
Yup, all you have to do is manually type out the tag!
As far as I know, neither Mbin nor Lemmy allow for filtering posts by a tag. Mbin does let you search everything for a tag, and it automatically does that when you click a tag.
I rely on the ten tags I've selected being unique enough that no one in the Fediverse outside this community will actively use our ten tags. I have some collisions with outside posts when using the windpower tag, but they're few enough that searching by the windpower tag still works well enough. I changed the old subreddit's space tag to microgravity on Mbin to reduce tag collisions with other people's pretty photos of space.
In the old days of Reddit, Reddit couldn't filter a subreddit by a post flair either. Crafty CSS writers basically made different "language" variants that make all posts invisible except the ones that contain a certain post flair. I shouldn't have to do that here, but it's an option.