You can do this by using a java action. One of the matching overloads of
should help you out, for example:
this.addOpenFormFeedback("MyFirstModule.HomeForm", "Formtitle: 3 + 2 = " + (3 + 2), ...)
I have wondered about the same option and have filed a feature request as has others. The request now has the status Scheduled for backlog. So I hope it will become available in a future release.