APM W3C Local Logging iRule

The HTTP::username command is used to retrieve the username from an HTTP Basic authentication request (Header: Authorization: Basic [bsae64-encoded user:pass]. So you would necessarily need to prompt the user for Basic authentication in order to get this value:

if { not ( [HTTP::header exists Authorization] ) } {
    set body "Authentication FailuredError: Authentication Failure"
    HTTP::respond 401 content  $body WWW-Authenticate "Basic realm=\"[HTTP::header Host]\"" Connection Close
}