Prevention Needs and Target Behavior Preferences in an App-Based Addiction Prevention Program for German Vocational School Students: Cluster Randomized Controlled Trial - Diana Guertler; Elaine Kraft; Dominic Bläsing; Anne Möhring; Christian Meyer; Hannah Schmidt; Florian Rehbein; Merten Neumann; Arne Dreißigacker; Anja Bischof; Gallus Bischof; Svenja Sürig; et al. (JMIR mHealth and uHealth)