PopIn doesn't sell tickets, but if your event is ticketed somewhere else (Eventbrite, Humanitix, a Stripe page, a Google Form), you can attach the link so guests buy in one tap.
Add the link
Open the Details tab
When creating or editing the event, go to the Details tab.
Tap "Add dress code or ticket link"
This reveals the optional fields. Paste your ticketing URL into the ticket link box.
Once saved, a clear "Get tickets" button appears on the event page for everyone who can see the event, including no-account guests on their invite link.
RSVP and tickets are separate. A guest can RSVP Going on PopIn and still need to buy a ticket through your link. Mention that in the description so nobody assumes RSVP = ticket.
The link opens in a new tab and is shown as-is, so double-check it points where you mean. For free events, just leave it blank.