To insert a boot tee fitting, go to
Ribbon: DM HVAC->Ducts-> 
Boot Tee
Pulldown Menu: DM HVAC->Fittings->Boot Tee
You will be prompted to specify the duct on which a boot tee is to be inserted.
Select duct or label to insert fitting at:
A boot tee will be inserted on the drawing, as shown below.
