In order to set your trade offer url you will have to follow these steps:

1. Go to your scrap.tf settings
2. Go to where it says "Trade Offer URL" and look at the text that can be found under the box.
3. Click on "Trade URL for Third-Party-Sites". It will open a steamcommunity.com page.
4. Scroll down until you see "Third-Party sites" - the URL under it is your trade offer URL.
5. Copy the URL and paste it on the box mentioned before (in your scrap.tf settings), then click the save button.

That's it! You have now set your trade offer URL and the bots will be able to send you a trade offer without adding you.

WARNING: If you have Steam Guard disabled or recently activated it for your browser, then you might be unable to accept trade offers from your webbrowser for a while.
You can still use your Steam client to accept trade offers like you always did. Don't know how to do that? Click here for a guide!