Azure API Management Custom SignUp Field

I am looking to add a custom signup field in Azure API Management but not able to figure it out where and which place.

Things i want to do:

• I have a set of apis i want to expose via API-M
• I want to add extra field say customer identifier while signup how can i add this extra field and monitor its value from developer portal, based on this value i can allow product subscription or reject it.

I have gone through this link but I am not clear and the link also says about deprecations.

Azure API Management – Additional fields during signup process