Greg,
I put this in my Transitions tab. detailedChangeDescription is a global String variable in the workflow. But serves as an example for what you're after. In the example, I am checking to make sure the string length is at least 10 characters. You could check for only 1 if you wish - or use whatever logic you want to.
wt.workflow.work.WfAssignedActivity activity = (wt.workflow.work.WfAssignedActivity) self.getObject();
wt.workflow.engine.ProcessData actData = activity.getContext();
detailedChangeDescription = actData.getTaskComments();
if (detailedChangeDescription.length() < 10) { throw new wt.util.WTException(new Throwable(new String("Please enter a detailed description in the Comments field.")));
}
This is a leverage of these 2 articles:
https://support.ptc.com/appserver/cs/view/solution.jsp?n=CS185827
https://support.ptc.com/appserver/cs/view/solution.jsp?n=CS182596