Irule insert html attribute, APM

Another option would be to use a stream profile on the VIP, and do a custom replacement. Below is some code to help with that. You'll want to modify the HTTP_REQUEST check for the URI to whatever you're looking for. But this worked for me when I tested it.

Basically, it will have the profile find matches where there's a tag and then when it finds it, in the STREAM_MATCHED event, if there's no action attribute, it will add it.

when HTTP_REQUEST {
    if { [string tolower [HTTP::uri]] contains "/Whatever/You/Are/Looking/For" } {
         Remove the Accept-Encoding header so that the server will not compress the response 
           (otherwise the stream rewriter won't work, since it doesn't decompress the stream)
        HTTP::header remove "Accept-Encoding"

         Set a variable so we know to do custom stream rewrite
        set strReplace 1
    }
}

when HTTP_RESPONSE {
    if { [info exists strReplace] && $strReplace && [HTTP::header "Content-Type"] contains "html" } {

         Look for any form tags
        set expr {@]*>@@}

         Disable the stream profile so we can manually update it
        STREAM::disable
    }
}

when STREAM_MATCHED {
     Log the match that was found
    log local0. "    Match: '[STREAM::match]'"

     Manual replacement to add the action attribute (since we couldn't check in expression)
    if { not ([STREAM::match] contains "action=") } {
        STREAM::replace [string map {"