Once you have registered and logged in, go to Yahoo! Canada Classifieds.
Click on the "Post Ad" link at the top of the page. Post your ad as you would like it to appear. Then, enter the web address in the "Web site URL" field of the ad submission form. Be sure to enter the URL exactly as it appears in the browser or it will not work.
If you have already submitted an ad, you can access this form by clicking the My Ads link from the Classifieds home page and then clicking the "Edit" link under the ad you wish to change. If you do not currently have a web page, you may set one up through a free home page provider like Yahoo! GeoCities.
The link will show up online in your ad when you click on "More Details" at the bottom of the ad.