First, you need to have the right profile settings on your Steam Account.
1: Go to your Steam profile and click on the "Edit Profile" button on the right side.
2: Then click on the "My Privacy Settings" on the right side.
3: Under "Profile Status" and "Inventory", choose "Public – Viewable by anyone on the World-Wide Web".
4: Then click the "Save Changes" on the bottom of the page.
After that please open your steam trade offers page:
At the sub-item "Third-Party Sites" you will find your Steam Trade Offers URL.