iRule to log a specific field in POST data

I'd like to have an irule log the authentication attempts for a web application I have that takes POST data. How do I pull a specific POST parameter out of the http content?

 

Suppose I have a login page, http://example.com/loginform.html" that will have POST data for "username" and "password". So, right now, I can detect the presence of the 'username' parameter in my submitted form by looking for HTTP::method as "POST" to HTTP::uri "login-form.html" and checking that [HTTP::payload] contains "username". But I'd like to be able to generate a syslog saying "User $username attempted login from [IP::client_addr]:[TCP::client_port]"

 

How do I best extract that "username" value from the payload data for logging without altering the actual transaction at all?

 

Thanks!