Forms personalization - passing a form field to the parameter of a concurrent request

I am using forms personalization to allow the user to submit a concurrent request from the Purchase Order form.  The concurrent program has one parameter - PO number.  I would like to pass the PO number from the form to be the default value for the parameter.  I know how to setup a global variable and set it to the PO number from the form.  How can I use this global variable as the default value for the parameter, or is there another way to pass the PO number from the form to the concurrent program?  Thanks.