<system-properties>
    <property name="season" value="spring"/>
</system-properties>

Option 2, when the server is running, run jboss-cli.sh or jboss-cli.bat command to add system property. The property will be persisted to standalone.xml, so it has the same effect as option 1:

jboss-cli.sh -c "/system-property=buzz:add(value='This\ is\ buzz')"

Option 3, add -Dkey=val to standalone.sh command line:

standalone.sh -Dseason=spring

Option 4, pass a properties file containing desired properties to standalone.sh. This is especially convenient if you need a list of dynamic system properties.

standalone.sh -P /tmp/a.properties

Now after the server is up and running, how do you verify your system properties are present with correct value?

• Test it with any deployed app that prints out the system properties. 

• View $JBOSS_HOME/standalone/log/server.log, search for the system property name.  Only system properties specified in command line (option 3 & 4) are logged at server startup. 

• View all system properties in admin console at http://localhost:9990.
• Runtime tab | Server | Configuration | Environment Properties

• To list all system properties present in standalone.xml (not including those specified in command line):
  jboss-cli.sh -c "/system-property=*:read-resource"