Hi
Ajay is right. SAP doesnt provide any field for entering Purchase Requisition number. You can make use of existing fields like assignment or text. You can also activate fields Reference specification 1/2/3 (you will find under general data in FSG), if these fields are not being already used for some other purpose.
Identify all those G/L's which would require Purchase Requisition number and create a separate FSG for them. So, for these set of G/L's you know that a particular field is being used to enter the purchase requisition number when you look at the FSG
Regards
Sowmya