Default Order header DFF with a value from SR when the order is created from Service request field s

We have a requirement that SR incident address should be populated on Order header DFF when the SR is created from Service request.

How to use defaulting rules to achieve above requirement.

I have gone through Note - How Does One Pass Descriptive Flexfield Default Values from Telesales to Order Management? (Doc ID 431070.1)

but could not understand how to pass order number to custom code and populate the value in DFF of order header.