global variable in concurrent program parameter

I used forms personalization to allow a user to submit a concurrent request from the Purchase Order form.  The form passes the purchase order number to the concurrent job using a global variable.  The default value for the parameter is: select :GLOBAL.XXKNC_PO_NUM FROM DUAL.  I would like to put this same concurrent program on a menu for users to submit and have the parameter default to null.  How can I change the setup of the parameter?  If I run it as is from a menu, it gives me an error saying that the global variable does not exist.  Can I