Hi,
For this, you will need to register on our AC API Developer site: archicadapi.graphisoft.com/. When you click on any menu on the site, eg API Reference, you will be asked to Register as Developer. When registering, after providing your personal information, you can order the single commercial license - here you will see the Registered API Developer Agreement where you can read detailed information about the license.
Best regards,
Ildiko