You use
Connect Into to automatically add a component to a system and create ductwork between the new component and the existing system.
For example, to add an air terminal to an existing system, place an air terminal in a view containing an existing duct system.
Layout panel
Connect Into. If the new component could be connected to more than one system, the Select Connector dialog opens.
The cursor is displayed with a plus sign.
The new component is added to the system.