Are you looking for Third Party Order Process or Free Goods Sales via Third Party system? I am sure, you should be knowing difference between these two. The link you provided explains how a Free Goods Sales scenario can be addressed via another interface.
I presume, you are looking for Third Party Order process with Free Goods scenario. If this is correct, then have a look at the following thread where similar topic has been discussed.
Third party free goods functionality
G. Lakshmipathi