BUI copy a workspace value on button click

Summary

Looking for a method to copy an unsaved value to a custom field

Content

Hi everyone,

Let me describe the business requirement. 

A new incident is in the process of being created. The agent selects a contact to be the primary contact. This contact's address display in the incident workspace. We have a button "copy contact address" that will use the contact's home address as the value to fill a custom field that stores the incident address. Using the BUI extensibility framework, I am only able access the field value of an incident that has already been saved. 
My question then is how can I access the contact's address without first saving the incident? The code below returns null unless the record has been saved (which I'm assuming is intended).