How to organize the binding of custom domains through CloudFlare?

Question

API

Users need to be able to attach their own domain. Work through CloudFlare.
How to implement it?
In fact, there is an API method for this to add a .
But what should be the algorithm of actions in the future, so that the service understands which URL to work with?
Need to specify an entry in CloudFlare that will be used to interact with a specific page? If so, which one?