See OH 4.1: Tutorial to manage custom semantic tags for how to add it using a .yml file. There is an issue and I think a PR open to add it to the docs, not sure what the hold up is.
But it’s important to remember that “regular” tags can only be added on the Item page when you click the “edit” icon under “non-semantic tags”. These tags can be anything.
But I’m this case it makes sense to create a new curtains semantic tag.