The Lambert Charter is a decision available to 
France.
In order to Take the decision France needs to be a Great power, have researched Ideological Thought and 
Madagascar needs to be not civilized and either in France sphere of influence or in no sphere of influence at all.
It increases France's influence in Madagascar with 150, which effectively puts it in France's sphere. It leads the way to the decision End the Merina Monarchy, where France annexes Magadagscar.
The ingame text states: The Lambert Charter was a document used by France to justify its many attempts at making inroads into, and eventually annex, the independent country and island of Madagascar. The document was signed by crown prince Radama II, who had grown up under the influence of French expatriates on Madagascar, and purported to gvie France the right to extract minerals and natural resources from the country, in return for a 10% royalty payable to the Royal House of Madagascar.