Hello Sonia,
I do not think user-exit is available for this requirement. So you or your ABAPer should be able to achieve this requirement through Implicit enhancement point in the program SAPLMIGO. (field GOITEM-KZEAR).
In case the issue is to prevent users to leave it blank, then it is very easy to achieve through tcode OMJX. Here set Radio button setting under Req. opposite to field GOITEM-KZEAR. After this setting no material document will be posted until this indicator is set.
Good luck
KJogeswaraRao
