Based on the manual, the accessory port in the back is a 15 pin D-Sub connector that looks those used in old VGA monitors.
Relevant signals:
Digirig audio cable will look like this:
D-Sub Pin 1 (AF IN) → 3.5mm TRRS ring 1 (RIG_AFIN)
D-Sub Pin 2 (AF OUT) → 3.5mm TRRS tip (RIG_AFOUT)
D-Sub Pin 3 (AF GND) → 3.5mm TRRS sleeve (GND)
D-Sub Pin 6 (EXT PTT) → 3.5mm TRRS ring 2 (RIG_PTT)