Life is easier now. 

When you are posting in a group, if you have already clicked on a "highlighted topic" from the right-side menu, the post you are publishing will be automatically tagged with that topic.  

Of course, if you want to edit the topic or remove it, you can do it any time after it is posted. 

Also, when you click on a highlighted topic from the right-side menu, the group feed will be filtered in a way that you see only the posts related to that highlighted topic. 

How to add a highlighted topic to a group? Click here to learn more.