Access your Audiences tab and click on the Audience Creation button in the bottom right corner.
Tip! When you create a new Opt-In Audience, you create an opt-in you can include on any of your registration forms. If you create an opt-in on your registration form, you will automatically generate a new Opt-In Audience within the Audience tab.
This is what will appear on registration forms within your contests and interactive content.
Starring the opt-in will make it appear in your starred items within your registration pages. It will also pin the audience to the top of your Opt-In audiences so you can easily access it.