In my experience, you have to find the community on web first (by searching https://lemmy.ml/c/tf2 in this case), before it appears in the app.

However, someone else (which you might not be seeing, since they are posting from an instance yours blocks) is saying you should search for https://lemmy.one/c/tf2@lemmy.ml and it should work.

Remote community discovery is definitely something Lemmy still has to improve in terms of UI/UX. @nutomic@lemmy.ml @dessalines@lemmy.ml maybe add some form of dedicated place to search for remote communities? Or maybe make the regular search endpoint understand more community formats, like !name@server, instead of just the URL.