1. Appearance, Menu, create new menu, select “top navigation”
2. Open “Screen Options” upper right next to “help”
3. Select “link target”
4. on left “add menu items” select “custom links” put your advertiser link in and name it. add it to the menu.
5. on the menu side, open custom link and select “open link in new tab”
6. Save menu.
Hope that helps.