It's not typical to adjust the inventory of a product you haven't yet created, but QBO will allow you to create an inventory product and then immediately adjust it. Not knowing exactly what you are trying to do, I'm not sure how to advise you beyond that.

If you want to try things out without messing up your books, I'd recommend signing up for a free developer account. That gives you access to up to 5 "sandboxes", basically dummy company files that you can try things out in. After I asked my initial question and got the first answers, this is what I did to see how it all worked before actually using it in my live books.

Here is more information on QBO sandboxes:

https://developer.intuit.com/app/developer/qbo/docs/develop/sandboxes#launch-a-sandbox

In the top right corner of that page is a link to sign up for a developer account too.