So what on earth is this site for? You can send a message. But to whom? Turns out that Non Dutch English speakers can use this site to sent "good wishes" to the new Royal couple in the Netherlands, when King Willem takes over from his mother on April 30th.
Seems the Dutch Ministry of Foreign Affairs forgot to add the instructions. Which explains why no-one has yet used the site. Only a photo of Minister Lilianne Ploumen opening the site together with the Director of Google Benelux is a clue that it comes from them. Missed opportunity.