I'll have to look on Monday when I get back to work for the exact set up in CPS. You will need to assign pins for push to talk, and that is done in CPS.
The Batlabs page for CDM1550 shows the pin out for the 20 pin connector on the back of the radio.
Pin 2 is external mic audio
Pin 7 is ground
You will need to assign a pin for the Push To talk.
On an RJ45 plug, the following pins would be used to connect the mic:
Pin 4 Ground
Pin 5 Mic audio
Pin 6 Push to talk
Pin 3 Hook
So, you would need to make a cable that does the following:
Pin 5 on the RJ45 would go to pin 2 on the 20 pin connector for Mic auto
Pin 4 on the RJ45 would go to pin 7 on the 20 pin connector for Ground
Pin 6 on the RJ45 would go to pin 3, depending on programming for PTT
Pin 3 on the RJ45 would go to pin x, depending on programming for the hook (HUB/hang up box)
(might be good to have someone double check this, as I've never personally done it on a CDM, but I have done it on a number of Kenwoods.
Ideally you would want some shielded cable for the mic audio, but you might be able to get away with a short run of Category 3 or 5 cable, or pretty much any multi conductor cable, for a short run.
I'll have to look at the CPS when I get to work tomorrow. I've got a MAC laptop I take home, and all my CPS/RSS is on a PC back at the shop.