I have a new rule that happens when a motion sensor gets triggered the siren goes on.
I want to start the rule with a button on the dashboard that I can turn on (Arm away) or turn off (disarm)
If you want to try to do it with RM instead of HSM you could have the motion sensor be the trigger action in the rule, and then use a conditional before sounding an alarm. Something like this... though my example is probably too primitive to be of practical value (and untested). HSM is really a better way to go.
If you go into HSM configuration you'll see options for arming and disarming. In my example I use security keypads either inside the front door or in the bedroom, but you can also just use buttons.