Asynchronously hit an external web service from an iRule?

If I may add, just be advised that making a sideband call on each request, for each user, may 1) create latency, 2) drive CPU a bit higher than you'd want it, and 3) require the extra effort of creating the sideband content itself. If, however, you absolutely wanted to do a sideband call and you don't care about a response, you could just use the connect and send commands, and set timeout and idle values for each to some very low numbers.