two ways to do that;
a) go to the community page, press the button 'post to community'..... and write the post
b) but the best way is just do everything like you used to do, just as the 1st tag use the tag of one community you choose - in this case, your post will goes directly to the community :)
Alien Art tag, obviously, is #hive-158694
was my pleasure to help you :P