Scroll down just a little further in the help documentation you linked to. There's a section in that endpoint documentation called Attribute Values that documents what attributes can be included in the request body. One of those is project-id. It's right between the sample request body and the sample response body.
If you look at below syntax for POST then I don't see here project id as parameter listed. I am trying to frame this syntax using CURL command. POST /api/api-version/sites/site-id/workbooks