You can used the Formats option on the Story List screen to see a list of the story formats (and their versions) currently installed in the Twine 2 application.you are running, and you can use the Change Story Format option on a particular story project's Story Map screen to see which story format (and it's version) is currently selected for that story project. I will assume you are using the default version of Harlowe, which is v1.2.4
You don't state how you are displaying the message just what action causes it to appear, so I am going to assume you are using a combination of a named hook and the (replace:) macro to display the message.
The following uses the (live:) macro to delay the execution of a (replace:) macro which updates the named hook with an empty value, it also uses the (stop:) macro to stop the (live:) macro after it has been triggered once.
(link: "Show Message")[
(replace: ?messages)[Hi there!]