Prerequisites
You must be:
An admin of the Confluence Server instance.
Either a technical contact or billing contact of the Confluence Server instance. To verify, login to MyAtlassian and ensure the Confluence Server instance appears in the Licenses page.
Guide
Click on the Confluence icon in the top-left corner.
Go to Apps > Find new apps.
When the Find new apps screen loads, locate Linking for Confluence Server. You may have to omit the term "Cloud" from the search to find the add-on.
Click Install (or Free Trial if the app requires a paid license) to download and install.
If the app requires a paid license, you will be provided with a minimum of 30-day free trial.Click Close in the Installed and ready-to-go dialog box.