For Triggering event enter the following:
{"data":"eyJ6b25lIjogImFzaWEtc291dGhlYXN0MS1jIiwgIm5hbWUiOiAieW91ci1pbnN0YW5jZS1ncm91cC1uYW1lIiwgInNpemUiOiAiMSJ9="}
This is simply the base64-encoded string for:
{"zone": "asia-southeast1-c", "name": "your-instance-group-name", "size": "1"}
@Daniel-svg List all instances (code example) in your project and resize them all.