Adding Structure Gadget to Confluence 8.x or Below

This article applies to Confluence 8.x or below. For Confluence 9.0 or above, see Adding Structure Macro to Confluence.

To enable Structure Gadget in Confluence, you first need to create an application link and configure that link.

The following Atlassian documentation will help you get started.

Unless you'd like to see Structure as an anonymous user, connect Confluence to Jira using Application Links. You'll need to enable outgoing authentication from Confluence to your Jira server.
Documentation: Configuring Application Links
1. Use OAuth Authentication to let the Confluence page viewer authenticate separately with Jira. (Preferred)
  Documentation: Configuring OAuth for an Application Link
2. Use Trusted Applications authentication if you'd like Confluence users to act in Jira under the same usernames without additional authentication.
  Documentation: Trusted Application Authentication
  
  Structure Gadget can be enabled to allow modifications to the structure, updating and creating issues under the account that is used by Confluence to access Jira. Make sure you understand how Trusted Applications work before allowing production structures to be accessed with this kind of authentication. Using OAuth is more secure, because the end-user will never be able to do anything that they are not able to do directly in Jira.
Add Structure Gadget to the list of External Gadgets. Remember that you can copy the URL of the Gadget from the gadgets selection dialog, when you click Add Gadget on the Jira dashboard.
Documentation: External Gadgets
Check the sample page to confirm whether you can include the Structure macro and get data from Jira.

Troubleshooting

If you have problems using the Structure gadget in Confluence, check the browser's console. If you see errors saying that loading some of the resources is denied, it is likely due to a CORS problem in Jira. To work around that problem, see Setting Up CORS Filter in JIRA.