@entonian It's in Japanese, but I've written a blog post on how to use it. The images are there, so it's a little easier to understand. Please take a look at it.
https://tootoise.hatenablog.jp/entry/2019/12/19/081000
There is no way to add a hashtag to the list. Sorry.
@entonian Sorry for the late reply.
There are two ways to add an account to a list.
1. “Settings” > “List Settings” > Tap “Your lists” > Tap the icon to the right of the navigation bar > Search for the people you want to add to the list and add them
2. Tap the icon to the right of the navigation bar on the profile view of the person you want to add to the list. > Tap "Add or remove from lists” > Select the list you want to add.