1. Log into your Jira instance as a user with Jira administrator rank.
2. Click on Jira Settings:
3. Next, click on Apps, on the newly loaded left-hand side panel:
and go to Find new apps.
4. Locate Issue Links Viewer via "search the marketplace" field - please note that its Vendor is Transition Technologies PSC:
5. Click Try free to begin a new trial or Buy now to purchase new license for Issue Links Viewer
6. Enter your information and click Generate license when redirected to MyAtlassian
7. Click Apply license