App vendors are not available for event payments. In this regard, I'd advise setting your payment option for the event to be cash / check only. Then in the event's description, mention that payment will be collected via the app you're wanting to use, and include vendor code / tag. Make note in your invites, or the description, or the confirmation thank you page / email that they'll need to send you the payment, with some kind of identifying info (such as email address used to register). Otherwise you can state that you won't be able to manually finalize their registration, by updating their payment status. You can still send them even registrant-only update emails up until they're paid, they'll just remain as payment pending status until you update them.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ William A Community & Social Media Support