Follow Hashtags
With Hubzilla, you can't just ‘follow’ hashtags as you would on other services. You can save a search for a hashtag and then access it... but that only brings up postings from the grid, i.e. from Hubzilla itself.
This is not very comfortable and also rather limited.
And the function cannot be easily integrated because Hubzilla works differently from some of the popular Fediverse services.
But Hubzilla wouldn't be Hubzilla, the all-in-one solution, if there wasn't an easy solution for that too.
If the administrator of your own hub allows it (i.e. has activated it), you can connect to RSS feeds. These are not the ‘connections’ that Hubzilla is basically known for, with the possibility of mutual interaction, but really just a feed. So if you connect to an RSS feed, the posts from the feed are added to the stream.
And this functionality is the starting point for following hashtags. Mastodon, for example, also offers feeds for hashtags. Their address looks like this schematically:
<mastodon-instance>/tags/<hashtag>.rss
The leading double hash (‘#’) must be omitted from the ‘<hashtag>’ part.
It makes sense to choose a more heavily frequented Mastodon instance.
So, for example, if you want to follow the hashtag #hubzilla and choose troet.cafe as your source, the feed URL is: https://troet.cafe/tags/hubzilla.rss.
If you now connect to this feed by entering the URL as the handle, postings with the desired hashtag will appear in your stream in future.
There is a small disadvantage to this way of following hashtags. The postings don't allow any kind of interaction. There is a link to the original post on the original Fediverse instance. You can click on it, but you may leave... well, usually, unless the post comes from a channel on your own hub... your own hub, and you can't interact there either, unless you have an account with the target instance.
But if you want to do that, the post has to be imported into the stream. This is a little cumbersome. You copy the URL at the top of the post and paste it into the search field for the stream. If you then press Enter to perform the search, the post will be imported into the stream and you can interact with it as usual... so ‘like’, ‘share’, ‘repeat’, ‘comment’... whatever is possible (and allowed).
Once again, a word in favour of Saiwal and his Utsukta themes, which I recently presented. As part of a discussion about importing posts from outside the instance, he quickly implemented an icon 
that appears next to such posts. One click on it... and the import is done. Great, isn't it?
So... you can follow hashtags with Hubzilla. You just have to know how to do it... and that's why I'm writing it down here!