You must be:
An admin of the Jira Cloud instance.
Either a technical contact or billing contact of the Jira Cloud instance. To verify, log in to MyAtlassian and ensure the Jira Cloud instance appears on the Licenses page.
Click on the Jira icon in the top-left corner.
Go to Apps > Explore more apps.
Search for "Automatic Issue Links for Jira (Beta)" and click Get app.
In the dialog window, click Get it now (or Try it free if the app requires a paid license) to download and install it.
If the app requires a paid license, you are provided a minimum 30-day free trial.
Once installation is complete, click Get Started to be redirected to the app page.