Alert callback needs a parameter

I’m looking for a way to add a parameter to a webhook callback on an alert generated by system data, e.g. “motion was detected.” I would like to create an alert rule to do this: https://path/to/api/motionDetected?IMEI=<imei_parm> but I haven’t found a way to specify / resolve a parameter from system info or data. The webhook works fine as long as I hard-code the imei number. I just need a way to pass in a variable.