You can do that too, but you can also take the whole URL with https:// and all and it will take it just fine. I prefer telling people that as copy/pasting is easier than telling them to edit it into a specific format

Have a look at this comment: https://lemmy.world/comment/1692

The steps you’ve taken to create and view your communities aren’t real clear, but the fact that you can’t view this unrelated community makes me think you’re not understanding the moderately confusing ways that remote communities hosted on another instance get replicated to the server your account is on. The TLDR is that:

• Remote communities don’t show up until a local user tries to interact with them.
• The first/best interaction to get things loaded is to search for the exact community string on https://lemmy.one/communities. The two ways to search are via bang-prefix like !askmidwest@midwest.social or url like https://Midwest.social/c/askmidwest. Confusingly, that search won’t return any results… which I find to be weird and bad. But it WILL invisibly cause your instance to discover the community in the background.
• Now that you’ve search by bang-prefix or url, the community you searched for will show up in https://lemmy.one/communities/. You can browse, though that may be inconvenient if there are a lot to browse through. Or now you can keyword search for something like askmidwest. NOW that will return a community result even though it didn’t before the bang-prefix/url search.

This is all pretty confusing, but predicable once you learn the rules… which are basically that you have to teach your instance about new communities by searching for them using the exact proper community string.

Maybe this isn’t the problem you’re having, but it’s my best guess based on your vague description.