Here's a short instruction on how you can do that.
How to get your Trade-URL (Browser Link):
You open this link http://steamcommunity.com/my/tradeoffers/privacy ;
You will have to log in your Steam-Account, if you are already logged in you can go to point 3;
You can see three different parts of your Steam settings. You scroll down to the third section of this site;
Now you can just copy a new link, and enter it in your GC account.
How to get your Trade-URL (Inside the Steam-App)
Go to your Profile;
Go to Inventory;
Open: Trade Offers;
Open: Who can send me Trade Offers? ;
The headline "Third party sites" allows you to check your current Steam URL and create a new one.
Now you can just copy this link, and enter it in your GC account.