It's not trivially represented in-game, but:
The original constituent provinces of the Republic of the Seven United Netherlands were the Duchy of Gelre, the Counties of Holland and Zeeland, the city of Utrecht, the Lordship of Overijssel, the free provinces of Friesland and Groningen, and the unrepresented (and untaxed) lordship of Drenthe.
The Peace of Westphalia assigned to the Republic of the United Netherlands regions designated as Staats-Vlaanderen (probably about half the in-game province of Vlaanderen), Staats-Brabant (roughly matching the game's provinces of Antwerpen and Breda), and Staats-Limburg (roughly matching the game's province of Limburg; the Treaty of Utrecht further yielded Staats-Oppergelre (the parts of the game's Gelre province that they didn't already have, AFAICT).
So basically, the in-game Netherlands formation event lets you build the historic 16th-18th century United Republic of the Netherlands, rather than the post-Napoleonic United Kingdom of the Netherlands (which further included the areas represented in-game as Luxemburg, Brabant, Liège, Ghent, and some more of the province of Vlaanderen).