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:
https://steamcommunity.com/id/me/tradeoffers/privacy#trade_offer_access_url
At the sub-item "Third-Party Sites" you will find your Steam Trade Offers URL.