Insert text to HTTP header via iRule event during APM execution

HTTP_REQUEST_SEND is also one of the event can be used to send the APM variable.

https://support.f5.com/csp/article/K74392192

so worth trying below. modify as needed.

when ACCESS_POLICY_AGENT_EVENT {
    if { [ACCESS::policy agent_id] eq "event_1" } {
        ACCESS::session data set session.custom.header "value"
        set header [ACCESS::session data get "session.custom.header"]
    }
 }
 
 when HTTP_REQUEST_SEND  {    
    clientside {
    if {[info exists header]}{ 
	HTTP::header insert "headername" $header
    } else {
	 return
	}
   }	
 }