insert login credentials using irules

there are some solutions... what is your requirement?

Do you want to authenticate without redirection to /my.policy (example, authentication for web services)? --> solution is enabling "clientless mode"

Do you want to manage authentication from irule but enable VPE evaluation for some other requirements like Host checker? --> you can provide session.logon.last.username and session.logon.last.password in ACCESS_SESSION_STARTED event.