ERROR MESSAGE

Hi All1

I had added a custom field onto a SAP standard GUI(subscreen).Now my requirement is to issue a error message when ever the user does not input values through list down box into the custom field with message suggesting him to enter values.Also it is required not to forward the transaction until the user inputs the value.

Please advise in which part of the standard program i have to coding to accomplish this.

Regards

Praneeth