Currently, you can only add a single promotion code to a single ticket type in the web portal. On the Tickets Price Add/Edit page, you will see Require a Promotion Code field. Enter the promotion code that you wish to add to that price.
You will need to create a new Ticket/Price type for each new promotion code in Web Portal. For more information about how to create prices in Web Portal see Building a Show in Web Portal https://support.agiletix.com/hc/en-us/articles/115000630106-Building-a-Show-in-Web-Portal- or Building an event in Web Portal Portal https://support.agiletix.com/hc/en-us/articles/115000661526-Building-an-Event-in-Web-Portal-