In order to establish a secure connection between JIRA and TFS4JIRA Synchronizer one has to:
With these information in place TFS4JIRA components can establish secure HTTPS connection between each other.
Please note, that encryption password can be choosen arbitrary (in particular - it is not your JIRA password). It just has to be the same in JIRA and TFS4JIRA Synchronizer.
If you have JIRA Cloud, you will need to expose the TFS4JIRA Synchronizer to the Internet, allowing incoming connections to it, either directly, or through a proxy, or by port forwarding. See Required network configuration for more details.
In JIRA administration go to Add-ons tab and page "TFS4JIRA Configuration". Enter Synchronizer URL and encryption password, then click "Test". If connection is successful click "Update" button.