Hi moretolearn,
If you have the store window setting enable in your Auctiva account, then any listings you post(even if its not through Auctiva), will be automatically be appended with the store window once the listing is imported into your Auctiva account. This typically takes 6-8 hours from when the listing is posted.
Or, you can insert the store window html into your item's html description when creating your listing through another service, and the store window will appear within the description exactly where the html was placed. Your store window html can be found at Auctiva -> Store(Tab) -> Store Window HTML.
-Mike