Document number flow to report

Hi All,

NAV 2015 here...
I made a copy of the purchase order report (10122) and put in our custom range, added it to the page 50/reports section and promoted it to Process. How do I get the document number to flow into the report automatically like it does on the original report. By that I mean, when I go to print the PO, it automatically picks up the PO number I'm on to print.

Thanks!