F5 APM Custom Variable Assign

Dear Dario,

Thank you for your suggestion. What i am trying to do is, we have a web application and i am trying to implement otp using apm. As APM doesn't have any option to do HTTP query to get mobile, we had created an api which will reply back with mobile number in the host header "X-MOBILE_PHONE" during the http auth call. I am trying to extract only mobile number from the session.http.last.response_header.0 and map it to a custom mobile attribute. I understand that we can do this with irule but i am not aware how to extract this field only.