Nagios acknowledge email, how do I add the user acknowledging it to the email

Hello,

I'm trying to add the user name of the person acknowledging a service check. Right now if you acknowledge a check you get a email like this:

From: nagios@abc.com [mailto:nagios@abc.com]
Sent: Thursday, January 03, 2013 12:32 PM
To: Me
Subject: ACKNOWLEDGEMENT:Server-name/Ping Check is CRITICAL

T:Thu Jan 3 12:31:43 EST 2013
I:PING CRITICAL - Packet loss = 100%

I want it changed to add the last line:

From: nagios@abc.com [mailto:nagios@abc.com]
Sent: Thursday, January 03, 2013 12:32 PM
To: Me
Subject: ACKNOWLEDGEMENT:Server-name/Ping Check is CRITICAL

T:Thu Jan 3 12:31:43 EST 2013
I:PING CRITICAL - Packet loss = 100%
ACK by:User-acknowledging-this-check

In my /etc/nagio/commands.cfg i have:

define command {
command_name notify-service-by-email
command_line /usr/bin/printf "%b" "T:$LONGDATETIME$\nI:$SERVICEOUTPUT$" | /bin/mail -s "$NOTIFICATIONTYPE$:$HOSTNAME$/$SERVICEDESC$ is $SERVICESTATE$" $CONTACTEMAIL$
}

What would I add or am i looking in the wrong place?