Architect Data Action POST to Lambda returns 403 Forbidden

Trying to trigger an AWS Lambda from an Architect Data Action via HTTPS. The integration keeps failing with a 403 Forbidden. I’ve set up the IAM role with lambda:InvokeFunction permission. The request body in Architect is just {"action": "update"}. Is there a specific header or signature required by the Lambda URL that I’m missing? The endpoint is a public Lambda URL.