How to add a filed in page of "Purchase Order"

Summary:

I am new to Oracle Cloud Application and I am doing some test in lab env. I would like to add a filed in page of "Purchase Order" and section "General".

Content (required):

I searched some trainings and I guess I can achieve it in Application Composer, but I could not find any standard or custom object which is likely the one I can add a new field to in Application "ERP and SCM".

Comparing with the information in "General" section, there are only a few field in Custom Object "Order" under "CRM Cloud". And I am not sure if a page for Procurement should be linked to a object under CRM.