To post a URL, type the ENTIRE URL starting with http.
Example: http://www.myredbook.com

If you are asking about a particular ad, most sites use Frames which hide the actual URL. To see the ad's URL

Internet Explorer:
1. Right click on the ad background - anywhere but a picture or link
2a. Select Open Frame in New Window
Copy and paste from the URL in the newly opened window into your message.
2b. Or, Select Properties
In the window that pops up, copy and paste from the Address (URL) field.
2c. Or, Select Create Shortcut
Double click on the shortcut to view the page and then copy and paste the url from the Address window

Mozilla / Netscape:
1. Right click on the ad background - anywhere but a picture or link
2a. Select This Frame -> Open Frame in New Window
Copy and paste from the URL in the newly opened window into your message.
2b. Or, Select This Frame -> View Frame Info
In the window that pops up, copy and paste from the Address (URL) field.

If using a Mac:
1. Hold down the shift key as you click to get the menu that comes up on a right click by those poor windows users (based on the instructions above).