The PTT Button is nothing more than the button you press to make your calls. In addition to the main button inside the call screen, you can configure a physical shortcut button to make a call with the screen locked or using the headset microphone.
To configure it, go to menu and select "Settings"
Select "PTT button" and then click on the first option, called "PTT shortcut button"
You will see this screen without a pre-defined PTT button. Connect your headset to the device.
Press the button on your headset.
And ready! Your phone will be configured as a PTT button. You can test your button in the "Test audio" functionality, that first contact on the personal account list.