The Hanseatic League is a very different type of country, as being a Building Based one. That means that it holds no land, and in case it would acquire any, it would also become a Settled Country. This means, that it can only expand via foreign buildings, from which its money and resources depend. This is due to the Hanseatic Merchant Federation, which unlocks its basic content and mechanics:

The other means that it has to expand its power is by a unique type of Subject, the Member of the Hansa. Only countries with at least one town or city, and having fewer than 5 locations, can be made members - an action that can be performed either through diplomacy, or by a war and a peace treaty. There are only 3 at start, Hamburg, Lübeck, and Bremen, and here are its rules: