eCATT Chain of Transactions

Hi Gurus,

I want to chain va01 and me21n. purchase requistion number will be at sales order schdule line data. by taking that number i have to create a purchase requisition. i can do chaining by taking number from the messages with the help of &ms1, &ms2 etc. How to do chainging by taking data from sales order shedule line. That purchase requisition number will be in grey mode and it will appear only in sales order change mode(va02).

thanks in advance