How can I create a trade at Groupees?

Hey guys,

Do you know about trading at groupees? Imo, it's a really cool thing! However, I do not understand how I can create a 3rd party trade, so that I upload a steam key that was bundled not at groupees but at some other site (HB for example). I know it is possible because there are a lot of trades that say 'Steam (3rd Party Key)' key but I cannot find a proper option to do it. Can you help me please?