The variable for putting the tracking number into an email is:

"<#TrackingNumber>"

You can also use it to put a link in an email with code like this, which is for USPS tracking:

"http://www.framed.usps.com/cgi-bin/cttgate/ontrack.cgi?temp=output-mm2bv.htm&tracknbr=<#TrackingNumber>"