Quick answer
You can find your trade URL in your Steam inventory, under your trade offer settings
Your Steam inventory must be public, or sellers can’t trade with you
Steps
Log in to Steam.
Open your Inventory.
Select Trade Offers.
Select Who can send me Trade Offers?
Copy your trade URL from the Third-Party Sites section.
Set your Steam inventory to Public. If it’s private, Eldorado sellers can’t trade with you.
What happens next
Paste your trade URL when Eldorado asks for it, for example at checkout. The seller uses it to send you a trade offer on Steam.
Troubleshooting
Sellers can’t send you a trade offer
Check that your inventory is public. On Steam, go to your profile, select Edit Profile, then Privacy Settings, and set Inventory to Public.