The API is a project module to enable in the studio
You must host those 2 files and let us know their web address so we can set them up in our internal system.
You can register and listen to particular events such as when the experience starts, or when an image marker is found/lost.
You can also setup your own events with the XR+ studio action editor. By doing so you can setup an action on a composition item to call a function you have defined in your script.
Let us know on which project you would like to activate the API module and the url of the .js and .css you need to load in the <body> of the experience.