The Onsite Advertiser Sign-up is an extension of Google site targeting that makes it easier for advertisers to bid on your site. Previous to the Adsense ad redesign, the ad units displayed “Advertise on this site” link that took advertisers to an informational landing page with details about your site and the Google AdWords advertising program. Now that link is gone, but you can add an custom link yourself.
Now you can create an “Advertise on this site” link outside of your ad units to sign up advertisers using Onsite Advertiser Sign-up. Follow these detailed instructions. Here is the format of the url you need to create –
client=NUMBER&referringUrl=http://www.YOURSITE.com">Advertise on this site</a>
Replace your publisher ID number with the “NUMBER” space shown above. (Your publisher ID is found under the My Account tab > Property info). Remember to include the ca-pub- in the ID. Replace your site’s URL with the “YOURSITE” space shown above. And simply place your link anywhere in the template.
You can customize the landing page such that advertisers see the logo, color scheme, and site description of your choice. Since there is a single landing page for all your multiple websites, you need to fix the landing page to protect privacy of your other sites.