The Authentication section of this page has more information about OAuth authentication, which is needed when the history of shortened links by end user needs to be managed and tracked.
Information about the API endpoints can be found here.
Within the Bubble editor, each call can be specified as being used as an "Action" or "Data".
Action lets you use the calls (like /shorten) via a workflow action
Data lets you use the call via the design tab (via the get data from external API expression) which can used to create a link on page load automatically
Choose "Action" to be able to use the call on the workflow tab, and choose "Data" to be able to use the call within the design tab of the bubble editor.
The "Shorten Link" API call in the sample app is used within a workflow to shorten links that are inputted by users. If you right-click > copy you can paste the call into your own application and modify it to suit your needs.