First of all, it's really good to know that you're using Back4App, we're grateful <3
Even with the guides, are you trying to apply another API to test your application? If the answer is yes, then you may want to know more about our API Hub with developers that allow other devs to use the API configured by them.
How can I access the Public APIs?
1. Access the Back4App site; if you're not logged in Back4App, you'll check it at Header, named as 'API Hub'.
If you're logged, you can check this option at bottom from the page, like the image below:
2. Then, in order to access the page, you'll check a space to search the APIs available to use!
3. Click on Search and you'll see the API available to access. With this you'll can see the keys to use it :)
In this article, it is possible to check public APIs availability and how to find them.