The simplest way is to use something like this in the Input Validation event:
@If( @IsDocBeingRecalculated | @ThisValue != null ; @Success ; @Failure( “A value for this field is required” ) )
A more flexible way is to use the QuerySave event. You can check the field’s value, use MsgBox for messages, and use
Call notesuidocument.GoToField( fieldname$ )
to put the cursor back where you want the user to be. Just set Continue to False to prevent the doc from being saved if there are missing entries.