In order to be able to use Windowmaker, you're going to need to purchase it from its developer. To do this, you'll be asked to fill out a form on the application's website to contact the sale team. It's nothing too complicated: you just provide some general information about yourself and the company you represent, as well as select the exact product you're interested in. Once that's done, the sale team will give you a reply telling you how much the product costs and how to obtain it.
Moreover, from the application's web page you can also request a demo or ask additional questions.