Edit build script to use custom library?

Hello,
I would like to edit the build script of my Netbeans FRC project in order to use an external library I have found. I am not familiar with Ant, however in the build script it says:

“Some useful command line properties: … -Dutility.jars=utils.jar a classpath separator delimited list of jars to be included with the application”

This sounds like what I want, but I am not sure where to specify command line properties. Does anyone know the easiest way to make this work? I’m really lost here.