Help with DHCP reservation/MAC address of Pi? Duck DNS help?!?!?

Super new here and I’m struggling so hard right now. I have a Mac that I’m trying to connect to Hassio via Duck DNS. I have an AirPort Extreme router. I am trying to make a DHCP reservation, but I don’t know that MAC address and I’m so lost on what to actually enter in the settings. Im so annoyed. I will attach the guides im following, but im really lost on how to relate that to what im trying to do.