There are some predefined variables that you can use in TEACUP:
{_siteurl}: The SiteURL variable in the Portal Object
{_page}: The subpath to the TEACUP page
For example, if {_siteurl} is "http://www.mysite.com/sub/" and {_page} is "fiction/thering.htm", the TEACUP page is at "http://www.mysite.com/sub/fiction/thering.htm".