I would imagine you use - DoScenarioConversation as the PreformAction then set your Conversation internal name as the StringParam:
You will need to set up a conversation as per the game rules, see one of the megaevents for an example. This is assuming Government xml files allow triggers, if not your screwed tbh.
Edit: Also as I do not have direct access tot he Intrigue xmls I can not say wether this event will fire as and when x government is chosen, or if the example above fires anytime you switched? Not sure tbh.