Choice screen auctions have been recently deployed in 31 European countries, allowing consumers to choose their preferred search engine on Google's Android platform instead of being automatically defaulted to Google's own search engine. I show that a seemingly minor detail in the design of these auctions—whether they are conducted on a "per appearance" or a "per install" basis—plays a major role in the mix and characteristics of auction winners and, consequently, in their expected market share. Furthermore, per install auctions distort search engines' incentives. Empirical evidence from Android choice screen auctions conducted in 2020 is consistent with my theoretical results.