Good morning, @maxproheating,
Great question! I'd be happy to help you. If I understand the question correctly, are you looking for a way where your customers can sign up for a membership to your service/product and automatically be charged monthly? If that's the case, you would have to create the recurring payment on your end; there is not a way for your customers to set that up themselves.
This article will walk you through how to set up recurring charges in QuickBooks Online for your customers:
Set up a recurring sales receipt
Hope that helps!