Once you're logged in, the iOS Dev Center main page should appear.
From the main page of the iOS Dev Center, open the Provisioning Portal:
Once on the Provisioning Portal, select the Identifiers link from the iOS Apps section and click on the Identifiers header again in the resulting screen to expand it:
Once the header is expanded, click on the App IDs link and click on the button in the upper-right corner with the plus + icon to register a new App ID:
In the screen that pops up, first enter a descriptive name for your App ID, usually the name of your app:
Next, scroll down and enter a bundle ID, a reverse-domain-name format identifier specific to your organization, school, or studio:
After you click on Submit, you can see that your App ID is enabled for Game Center:
What did we do?
We created a tag that will allow Game Center to correctly associate our requests with the right set of leaderboards and achievements, once we create that set.