Hi, welcome.
Assign first your "B" switch to one receiver output, maybe the channel6 that left.
Connect last input6 wire (assuming you are using PWM input with all wires) Sorry, Sbus
Redo the transmitter wizard and move quickly the switch when the wizard ask for Accesory0.
Now you can use the Accessory0 as Arming switch, be sure you set the Accessory0 neutral to middle.