Background: The success of Internet-based mental health interventions in practice -i.e., in-the-wild -depends on the uptake and retention of the application and the user's focused attention in the moment of use. Incorporating game-based motivational design into digital interventions delivered in-the-wild has been shown to increase uptake and retention in Internet-based training; however, there are outstanding questions about the potential of game-based motivational strategies to increase engagement with a task in the moment of use, and -as a result -improve intervention efficacy without accompanying increases in treatment exposure. Objective: Designers of Internet-based interventions need to know whether game-based motivational design strategies can increase in-the-moment engagement and thus improve intervention efficacy. We investigate the effects of one motivational design strategy (avatar customization) in an example mental health intervention (computerized cognitive training for attention bias modification). Methods: We assigned 317 participants to either a customized avatar or an assigned avatar condition. After measuring state-anxiety (STAI), we randomly assigned half of the participants in each condition to either an attentional retraining condition (ABMT) or a control condition. After training, participants were exposed to a Negative Mood Induction using images with strong negative valance (IAPS), after which we measured state-anxiety again. Results: Avatar customization improved treatment efficacy, evidenced as decreased posttraining state-anxiety when controlling for baseline state-anxiety, for those who trained attention bias modification; however, those who did not train experienced decreased resilience to the negative mood induction (F 1,252 =6.86, P=.009, η p