Involving end-users early in developing rehabilitation technologies is crucial, especially when designing solutions that aim to provide understandable and instructive feedback on activity or performance. This study explores user needs and preferences for a smartwatch and -phone application (app) to monitor and promote arm activity in stroke survivors. Semi-structured interviews with three stroke survivors and five healthcare professionals (HCPs) identified three key themes using reflexive thematic analysis: feedback, user interface, and motivational strategies. Both groups highlighted the need for an arm movement tracker, as existing commercial options primarily focus on lower limb activity, such as step counting. Stroke survivors preferred simple data representation and realtime tracking with daily and weekly summaries. Feedback was deemed essential, with stroke survivors valuing positive and corrective feedback to increase engagement. Participants emphasized the importance of personalizing the app, advocating for customizable features like font sizes, contrast, and feedback modes to meet varying needs. Moreover, remote HCP supervision was highlighted as a key motivational factor to promote accountability and support therapy planning. Further, incremental goal setting emerged as a potential key motivator. Together, these elements provide a roadmap for implementing more intuitive mobile apps to support arm rehabilitation after stroke.