You will find the URL address needed to complete the transaction by going to your inventory and clicking the "Trade Offers" button.
In the next window, select "Who can send me Trade Offers?" in the right-hand panel. The link is available on its left side. Select an address by clicking on it and copy it (ctrl + c).
Now you can paste the link in the appropriate field when buying the skin.
Note the message below the Trade URL panel: inventory Privacy must be set to Public in order to receive trade offers.