Validate quantity field on CO11N Program SAPLCOWB field ERFMG

Hi all, on the requirement Im trying to develop there is the need to validate quantity field so it can´t be 0 or empty. Already tried on BADI -- WORKORDER_CONFIRM but it seems not being able to do anything when saving.

The question is in which user exit could this enhacement go, so it can be validated at the moment the line is validated.

thanks,