iRule Redirect after WEBSSO is finished

Do you mean after WEBSSO is finished POSTING the credentials to backend application form?

If so, you have to wait for some indication from the server that the auth worked. You can anticipate that in HTTP_RESPONSE or HTTP_RESPONSE_DATA.

If on the other hand you want to intercept the POST before it is sent to backend:

You have an option to enable HTTP events, trigerred by requests/responses generated by the policy, via this command:

with access restrict set to disable, you have access to the regular HTTP events and you can try to use either HTTP_REQUEST_SEND or HTTP_REQUEST_RELEASE.