Howto: Prefill an attribute value based on another attribute (prior to clicking on [Apply])

When creating/editing a user object in Active Roles, I would like to be able to generate (let's say) the PhysicalDeliveryOfficeName attribute according to the value entered in the Description attribute. I don't want this to occur AFTER I click on [Apply] or [Ok] but as my user types in the description value,

In other words, as the Description attribute is being filled, I want a script to evaluate the description value and immediately fill the PhysicalDeliveryOfficeName - in similar ways I would achieve that using a "Property Generation and Validation" policy. "Property Generation and Validation" will simply not allow me to perform the evaluation I want.

I have inspiring myself from the "
How to create a Bound List-box with 4 Levels (4244019)" without success. 

Any help/sample would be appreciated.