The only two predicates available to create WAF Rules based on response is headers and status code:

I don't think you are able to do it based on specific message in response code, I would advise opening a support case to confirm that.
Regards,
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Sarvesh Lad
Tech Lead @ On-Prem Managed Services (WAF, DAM, DRA & Sonar)
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Original Message:
Sent: 05-12-2023 13:33
From: Jose Yero
Subject: Create rule based on response.
Hi Sarvesh.
The mode is Reverse Proxy.
I have mobile apps that have specific response messages and I want to map the events based on that response that comes out in the "Response" tab.

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Original Message:
Sent: 05-12-2023 12:03
From: Sarvesh Lad
Subject: Create rule based on response.
What is the deployment mode of the WAF?
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Sarvesh Lad
Tech Lead @ On-Prem Managed Services (WAF, DAM, DRA & Sonar)