Custom push-to-talk button on a handheld two-way radio
The documentation for your radio indicates that it is compatible with the Kenwood TK350, which uses two different TRS (tip-ring-sleeve) phone connectors for the microphone and speaker.

The following diagram that I found here suggests that the sleeve of the 2.5 mm connector is ground, while the sleeve of the 3.5 mm connector is PTT. Normally, you short the PPT line to ground to activate the transmitter.

http://members.shaw.ca/swstuff/kenwood-schematic.png