Hi!
Are you referring to the code editor of the site?
It’s simple! Click on the tab “Code” in your site editor and then click the “Turn on Developer Tools” button.
There you have it! on the bottom of the screen you got the code editor of the page.
(If that is not what you meant, please let me know and elaborate on your situation so we can find a solution for your issue)
As for integrating with 3rd-party-apps, you can use the Wix-Fetch method in order to call an out-source service but note that usually you’ll need to use the exposed API/key of the app/service itself in order to make it work in your site.
Hope it helps!
Doron. 