1. Log into your Jira instance as an admin.
2. Click the admin dropdown and choose Add-ons. The Manage add-ons screen loads.
3. Click Find new apps or Find new add-ons from the left-hand side of the page.
4. Locate Issue Picker Custom Field Type for Jira via search. Results include app versions compatible with your Jira instance.
5. Click Install to download and install your app. 
6. You're all set! Click Close in the Installed and ready to go dialog.