A URL to call/execute a node red flow?

So you mean it could be just a link on a web page? If so then rather than a link the guy coding the page has to make it a form with a submit button. In fact that can effectively just be the button wrapped in a form, and an html button doesn't actually have to look like a button.
If you want to do it from, for example, an email where the email viewer is converting the url text into a link then as far as I know you can't make that do a POST.

Yep - but internally we called them CookBooks !

Craig