This is a FAKE event. You will need to create a macro and turn on some settings in Maptool to emulate this.
This event can be simulated by turning on "Show stat sheet on mouse over" in the MapTool Preferences:
menu--> edit --> preferences --> Interactions --> 'Tokens' box: at the bottom --> check the checkbox
And then creating a macro as a campaign property e.g.:
If you create a lib token
lib:onMouseOver with the (self created) macro
onMouseOver() then this macro will be called EVERY time you hover your mouse over a token.
Note that this is extremely tricky and can result in system crashes or lock-ups if done incorrectly. The point is that the macro is called continuously when you hover over the token, so you have to take that into account when creating the macro.
You can download a drop-in here  (if the link directs you to the 'forbidden' page then right mouse click on link and copy link location and paste that in the browser) that shows two methods that use this event. One method will run only once, this can be used e.g. to setup a token after you dragged it onto the map. The other method will run every time you hover over a token (but not continuously)