Trade URL allows third-party websites, like DMarket, to send you trade offers. In order to find it, you need to navigate to the DMarket settings and click “Steam account”. There is a hyperlink that will lead you to exactly where your Steam Trade URL is stored (Fig. 1 and Fig. 2)
FIGURE 1
FIGURE 2
Copy and paste it into your DMarket account settings, click “Save”, and you’re good to go (Fig. 3)!
FIGURE 3
Important: if you create a new Trade URL, the previous one will be revoked and, therefore, invalid. In this case, you’ll need to paste the new URL into your DMarket account again.