Hello Alberto,
If there is no link 3rd party process how could there be the link with PR and PO
correct me if i am wrong.
As per my understanding user might have to create sales order by punching PO number manualy in SO if they are not using third party process. As he wants PO number in SO he
has to punch manualy in SO.
Please close the third if you have got the solution
Regards
Yogesh