Despite numerous health advantages wearable health trackers could offer, tracker users often abandon their devices after a short period of time. One reason for abandonment is users' lack of identification of their trackers as an accurate reflection of who and what they are (i.e., sense of identity). In this study, we examine the role of cosmetic customization (i.e., a modification of a given product's presentation) of a wearable health tracker in increasing one's sense of identity. Furthermore, we explore if one's sense of identity can explain the relationship between cosmetic customization and user engagement (assessed by attitude, exercise intention, and sense of attachment) with a wearable tracker. We conducted a between-subjects online experiment and found that individuals presented with a high level of cosmetic customization features experienced a higher sense of identity with a wearable health tracker, which in turn was associated with more favorable attitude, higher exercise intention, and greater sense of attachment towards the tracker than individuals presented with a low level of cosmetic customization features. Based on the study results, we recommend the designers of wearable health trackers to provide users with salient and recognizable cosmetic customization features so that users can establish a good "identity fit" with their tracker.