The Importance of Public and Private API Endpoints

I've been building applications and when I come into something that I cannot do myself, knowing how to build something from the ground up. So if I wanted to build a form with just HTML and I wanted to submit it to a specific endpoint, I want to know where that endpoint is. And some of them do give you a function that you can return or sure you can just use that function reference anywhere then in place of a string URL. That's just fine. As long as maybe I don't have to explicitly know where that URL is, but as long as I have some reference to it that treats it as a URL or a string, right?