You can use hashtags in your Community in the following steps:
1. Click on the Commerce tab on the left side bar.
2. Click on Community.
3. Click on the Community you want to edit.
4. Start a post by clicking on the top space by the + sign and start typing.
5. Add hashtags to the bottom of the post.
6. The hashtags are clickable which filters out all posts that contains the word.
7. Hashtags can also be used in comments.
8. Watch the video below for more information.