Boot Tee

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.