You sure can. When you put whether to already read/are reading/want to read it opens the little window, first blank space is for adding tags. You can write anything and it will turn into a tag
I can't do it anymore on mobile. You can probably do it on a computer/laptop though.
That makes sense thanks a lot
Had the same problem, but got recommended a keyboard app that works for tags
https://www.mangago.me/thing/about/687195/
as answer to my comment
I've a question that i've been meaning to ask for a long time that is how can i create new tags? Like i can only add existing tags or the popular ones, but i'd like to create my own ones. Can i do that?