Use property from teamcity.default.properties in labeling pattern

I'm trying to do something similiar, but I'm having trouble getting the parameter to parse properly
I followed the instructions for defining properties (http://confluence.jetbrains.net/display/TCD3/System+Properties+of+a+Build+Configuration) and made a file called teamcity.default.properties, with my custom property

env.MY_VAR=123

The problem is I can't seem to access it from within the configuration editor's General Settings tab.

When I try to use this variable in the General Settings tab, using %env.MY_VAR%, I get an error saying there is no suitable agent to run the build configuration. Presdumably because the web UI doesn't know this property is defined in the teamcity.default.properties file

I know the configuration can find the property correctly, since I can see it in a build log's parameters list. Note: I can successfully run the configuration when I remove the reference to %env.MY_VAR%

I've looked around for more information, but all I could find is reference to how to define a custom property in a text file, but nothing that clearly describes how to reference it in the configuration settings

Any help is appreciated

thanks
Michael