If you would like to create an Extras to be sold only with a specific Promotion Code, please follow this step by step guide:
Set up rate plans in the booking engine and link Extras to that rate plan
Then set up a new Promotion Code
Please attach this Promotion Code to the rate plan that you have just created
Please make sure to enable the option where the rate plan would be displayed only when a linked Promotion Code is entered, as per the screenshot below.