Hello,
It's more probably an intent than a button's keycode. Should you ask the manufacturer or check the unfiltered logs.
From that thread https://stackoverflow.com/a/40493624 I saw it's something like com.kodiak.intent.action.PTT_BUTTON or com.kyocera.android.intent.action.PTT_BUTTON, worse a try with the brand in your case. Good luck
Try these, they are industry standard, I have used them, it may vary from manufacturer to manufacturer.
android.intent.action.PTT.down
android.intent.action.PTT.up
android.intent.action.CHANNELUP.down
android.intent.action.CHANNELDOWN.down