In the event dashboard, click on "Registrations".

To create a new addon, click the “Create new addon” button to open the new addon form.

In the new addon form, select the tickets for which this addon is applicable.

Put in the title and description of the addon. If the addon is paid, ensure that the “Is paid?” checkbox is checked, and enter the amount.

Click "Save" to save the changes. This will create an "Add On" which will be available only for the selected Ticket type. 

How to create add-ons that the attendee can purchase?