I need alert opened date should be populate on detection time filed in the incident

[ashok17]

I need alert opened date should be populate on detection time filed in the incident, Please suggest how to achieve this requirement and i have tried below After Business Rule on incident table but not works:

if (!current.u_detection_time && current.u_alert) {
var alertRec = new GlideRecord('u_alert');
if (alertRec.get(current.u_alert)) {
current.u_detection_time = alertRec.u_opened_at;
}
}