iRule to add www without changing URL

Do you want to change Host header sent from the client in server side connection or do you want to redirect the client to  https://WWW.abc.com/testfolder/blahblahblah

to redirect the client, use following code

when HTTP_REQUEST {
    if {[HTTP::host] eq "abc.com"} {
        HTTP::redirect "https://www.abc.com[HTTP::uri]"
    }
}