Click the second icon from the left, with 3 lines. At the top of that page you can type in a search term. However, a couple caveats... 1, it won't find anything if you do the full community@domain, so just search on the community name portion. 2, it only returns 5 results, so if the one you're looking for isn't in that list your kinda screwed. 3, I noticed a bug where if you preview a community and then press back, it goes back to its default state, so you have to delete a letter and re-type it to get the results list again.

Adding from your instance's website might be easier in some cases, but I've found the feature in the app easier often times.