If the destination is going to be the same every time, program the button as Feature 11-10-XX. Operation is push on, push off.
If the destination is going to change, program the button as Feature 11-10. Operation is push on and dial the destination number, push off and dial 10.
The operator can also holler at someone to cover the phone, and that other person can dial Feature 11-10 and their own extension number at their phone to enable the forwarding, the operator can turn it off when they return to their phone.
Programming:
Feature 00
Press the button to be programmed
Press Feature, dial 11, dial 10, and optionally dial the destination extension
Feature 00 to quit