I would really like to have my smart home monitor get turned on and off via my arrival sensor.
Cannot find a setting to do that.
As a work around I created a automation that turns on the sirens if the arrival sensor is “away” and a intrusion sensor activates.
Is there a better solution?
Smartthings has changed their design philosophy with regard to security devices and systems , and this can no longer be done at all in the new app (“SmartThings (Samsung connect)“). 
You can do it in the Classic app using a routine, but there are two important notes:
One) they have already told us that the classic app will be going away, they just haven’t given us a specific date yet
Two) both the new app and the classic app have a feature called “smart home monitor” but these are two completely independent features and don’t work the same way and are not synchronized. So if you set up smart home monitor alerts in the new app, you will not be able to change them or dismiss them from the classic app and vice a versa. This tends to create a lot of confusion. 
The following thread lists the differences between the new app and the classic app. (The thread title is a clickable link)
Thanks!
I’m only using the new app, so I’ll just use my little “cheat” automation for times i just step away from the boat for a bit, vs leaving and locking up, and arming the security.