The only way I have found to trigger a recording from a webCoRE piston is using IFTTT webhooks.
example piston shown is very basic just to see if it would work and it does and creates a recording in the Arlo Library.
Switch 20 is a virtual ON/Off switch I created just as a test trigger for the piston
Motion Sensor 1 is my Arlo Camera
Obviously you need to set up the IFTTT url in webCoRE dashboard settings .
Then create an IFTTT applet using Maker Webhooks and Arlo.
It a work around solution but it works.
No option that I can see in IFTTT for capture though.