Installation [Data Center]

Log into your Jira instance as a user with Jira administrator rank.
Click the admin drop-down (cog icon on the the right-upper corner of the screen) and select Manage apps.
Click Find new add-ons from the left-hand side of the page.
Locate Issue Links Viewer via "search the marketplace" field - please note that its Vendor is Transition Technologies PSC.
Click Free trial to begin a new trial or Buy now to purchase new license for Issue Links Viewer

Enter your information and click Generate license when redirected to MyAtlassian
Click Apply license
The application installed correctly

Issue Links Viewer