If the application prompts the user to send the credentials in a POST request, you'd need to inspect the HTTP request payload--not the HTTP Authorization header value (like HTTP::username and HTTP::password do). If you just want to log the username and password, you could use a stream profile and STREAM::expression based iRule. Here's an example that is probably more involved than you need:
http://devcentral.f5.com/wiki/default.aspx/iRules/parse_username_from_http_requests_rule.html
If you want help adapting that to your scenario, can you provide a sample request with anonymized credentials? Can you also confirm the LTM version you're using?
You can get a sample request using HttpFox for Firefox or Fiddler2.
Aaron
Nimbostratus
