You can use the external effects bus as an insert effect or as a send effect, which is an insert effect on an effect channel track.
Prerequisite
Set up your external effects in the Audio Connections window.
Procedure
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Open the Select Insert menu.
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Select an external effect from the External Plug-ins submenu.
External effects are indicated by an x icon in the list next to their names in the Select Insert pop-up menu.
Result
The external effects bus is loaded into the effect slot.
A parameter window opens, showing the Delay, Send Gain, and Return Gain settings for the external effects bus. You can adjust these settings while playing back.
The audio signal from the channel is sent to the outputs on the audio hardware, through your external effect device, and back to the program via the inputs on the audio hardware.