irule to detect forms authentication

Im in need of a bit of help.

I have completing SSO via forms based authentication.  The response from the from is a json message, advising the client to redirect to a new URL.

When setting up the form, there isn't to option to inspect a json payload, but I am assuming I could complete this using an irule?

In short, I want to receive the following json:

{
"redirectUri": "https://url.goes.here/",
"data": ""
}

and redirect to a known url

If the json received is different to what I am expecting, I want to terminate the APM session.

Open to all ideas and suggestions.

Thanks