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.