src/main/java/org/imsglobal/lti2/LTI2Util.java
public static void addCustomToLaunch(Properties ltiProps, Properties custom) 
	{
        Enumeration<?> e = custom.propertyNames();
        while (e.hasMoreElements()) {
            String keyStr = (String) e.nextElement();
            String value =  custom.getProperty(keyStr);
            setProperty(ltiProps,"custom_"+keyStr,value);
        }           
	}
public static void addCustomToLaunch(Properties ltiProps, Properties custom) 
	{
        Enumeration<?> e = custom.propertyNames();
        while (e.hasMoreElements()) {
            String keyStr = (String) e.nextElement();
            String value =  custom.getProperty(keyStr);
            setProperty(ltiProps,"custom_"+keyStr,value);
        }           
	}
Place the custom values into the launch