To add a story format, you'll need to know its address. A story format should
include this in its documentation. A story format address must be a URL, with a
https:// in front of it.
Once you know your story format's address, choose Add from the Story Format top toolbar tab and enter it in the dialog that appears. If Twine is able to load the story format from the address you've entered, it will show a preview of the format name and version you're adding. Choose Add in the dialog to add it. It'll appear in the list of story format cards immediately. You can now either set it as the default story format or change individual stories to use it, if you like.