The warranty option is not set through the API. Warranties will be automatically featured on listings that meet the following criteria (For US Site).
1. Items in eligible Electronics and Home & Garden categories on eBay.com.
2. From sellers who are not considered below-standard and have at least 50 feedback.
3. All item conditions with the exception of "for parts" listings.
4. All selling formats except classifieds.
5. All items with a selling price of $40 or higher.
The eligible categories and the price limit can be different for different eBay sites.
Additional Information can be found at : http://pages.ebay.com/buyerprotection/squaretrade.html#q2