This only applies for Jira Cloud, because application links are created automatically for Jira Server / Data Center.
The Outlook app uses OAuth 1.0a to connect to your Jira instance. You need to create a new application link, so Outlook can connect safely. Please follow the instructions carefully - any misspelling could prevent Jira for Outlook from working correctly.
Open admin area
Login as Jira admin and go to: Settings → Products → Application links
Create new application link
Now create a new Application Link with this following URL and click on Create new link.
There will be a popup, telling you about the application link you are about to create. Just click Continue to get to the next step. Important: Leave the checkbox “I am an admin” checked.
Afterwards, you should be taken to a success screen. You can now go back to Jira and install the Outlook add-in.