Transition Tee
To insert a transition tee fitting, go to
Ribbon: 
Transition Tee
Pulldown Menu:
You will be prompted to specify the duct on which a transition tee is to be inserted.
Select duct or label to insert fitting at:
A transition tee fitting will be inserted on the drawing, as shown below.
Related Options
Angle of transitions: Sets the default angle of duct transitions.