You would set any system properties like that in the standalone.conf or alternatively in the standalone.sh/bat files or potentially on the command line. You basically need to pass that argument to java itself. You can also do it via command line into the runtime system.
Here are a couple of posts:
http://javahowto.blogspot.com/2012/09/how-to-add-system-properties-to-jboss.html
https://community.jboss.org/thread/197273
HTH