How setup signing works and what is LinkId

Hi everyone.
I am preparing own app for communicating with Pixhawk autopilot via MAVLink protocol and telemetry antennas. I created code for MAVLink 2 communication and everything is working well. Now I would like to append signature at the end of the packet and here is my questions:

  1. When I send SETUP_SIGNING message to the autopilot I should apply signature to this message?
  2. How do I know what channel I am using? Where can I find value which I have to put in LinkID (first byte of signature)