A Link Action is a workhorse. It points to any URL, including a website, blog, survey, app, or generally anything on the web. It's common to have multiple Link Actions on a page.
After selecting the Link Action, paste the destination URL in the URL box. The "Fetch" button will retrieve and populate the Preview metadata is available. You can edit it once retrieved, If the URL has no metadata, or if you want to change any of it, then enter new data in the appropriate fields.
Reference the Action Formatting and Controls FAQ for a refresher on the controsl that are common to Actions.
Hit [Save] add the Action to your Library, and then a second [Save] adds it to the App you are editing.