Check out the openEventDialog function, which also defines some window arguments down in line 434 if you want to do it manually.
/**
* Opens the event dialog with the given item (task OR event)
*
* @param calendarItem The item to open the dialog with
* @param calendar The calendar to open the dialog with.
* @param mode The operation the dialog should do ("new", "modify")
* @param callback The callback to call when the dialog has completed.
* @param job (optional) The job object for the modification.
* @param initialDate (optional) The initial date for new task datepickers
*/
function openEventDialog(calendarItem, calendar, mode, callback, job, initialDate)
You can call this function directly on the window object of the main Thunderbird window. This could for example be done by adding a XUL overlay to the window that has a script block that eventually calls this function.