User exit in CO02

Hi.. I neeed an user exit to save the material no in CO02 to a user field in the same CO02.

Since CO02 is only for change, can't we do it in the production order creation CO01, where this field directly gets updated in the CO02. The structure where the field needs to be updated is AFVGD-USR00. Wht needs to be done is just save the material no in the above mentioned field..

Can anyone tell me a user exit to use to solve this isssue?? Tnx