You can use the On click microflow/nanoflow event, in the Events tab, the entity type of the parameter that the widget will pass will be the object selected in the calendar, according to the return of your microflow that feeds the calendar.
With this parameter you can get the information you want, but according to the microflow source of the calendar.
You just need to define in the Events tab, if the on click will be calling the microflow or the nanoflow, so you can do your logic.
If needed, Mendix provides documentation for the widget: