OK, finally got things working like I wanted. I have posted a step by step description of the Use Case and steps to configure and automate over at the Arlo community at the following link: https://community.netgear.com/t5/Arlo/Record-without-motion-Smartthings-Arlo/m-p/1533359#M50491
Below is the basic outline:
OK Finally got everything working working and wanted to list out the specific steps for anyone else looking in the future. My Use Case was to trigger a one minute minute recording from an external motion sensor using Smartthings Smart Home Monitor. The motion sensor I am using is the SkyBell HD doorbell integrated into Smartthings. The specific Use Case is that when there is motion at the front door, record on a camera that will capture the person answering the door and if guests enter the house they will also be recorded.
Here are the specific steps to get it working:
Integrate the Arlo into Smartthings using the following: https://support.smartthings.com/hc/en-us/articles/209529506-How-to-connect-Netgear-Arlo-Cameras
In the Arlo App, set the Mode to Disarmed
In the Smartthings app, create a Smart Home Monitoring Custom Rule as follows:
Select Device to Use as Motion Sensor
Select the Skybell HD as the motion sensor (mine is named Front Door)
On the next page select Camera to record as the Arlo camera you want to use
Select Done and then finish the configuration. Press Save until you are back at the beginning. You should get a message that says Smart Home Monitor is updated.
Once this is done turn the camera On. This can be done in either the Arlo app or in Smartthings. I have a virtual switch setup in ST that I use to turn the camera on/off for ease of use in other automations.
Trigger the motion on the Skybell HD and you should have a recording start.
The main problem I was having was confusion about the Arlo modes and interactions with ST. Also I had an issue where the Skybell was not passing the motion event on to ST. I solved this by deleting the Skybell and re-adding. Once this was doen everything is working great.
Hope that someone else finds this useful in the future.