Background: New initiates on antiretroviral therapy (ART) are at high risk of treatment discontinuation, putting their health at risk. In low-resource settings, like Malawi, appropriate digital health applications must fit into local connectivity and resource constraints. Target users' perspectives are critical for app usability, buy-in and optimization. We describe the formative stages of the design of a two-way text-based (2wT) system of tailored reminders and adherence messages for new ART initiates and share results from key informant interviews with HCWs focused on app usability and acceptability. Methods: Using a co-creation approach with clients, clinical, technical and evaluation teams and over app development, we held four informal user feedback sessions, a small pilot with 50 clients, and ten key informant (KIIs) to deepen our understanding of healthcare workers (HCWs) needs, acceptability and usability. Results: Formative research informed the design of interactive client-to-HCW communication, refining of the language and timing of weekly text blast motivational messages and tailored client-specific visit reminders. Informal feedback from HCW stakeholders also informed educational materials to enhance 2wT client understanding of how to report transfers, request visit date changes and ask questions related to their visits. In KII, HCWs noted their appreciation for the co-creation process, believing that the participatory HCD process and responsive design team enabled the development of a highly acceptable and usable 2wT digital tool. HCWs also suggested future improvements to promote inclusion of clients of varying literacy levels and economic backgrounds as well as integrating with other health platforms to improve uptake of 2wT. Conclusions: Inclusion of HCWs increased perceptions of app usability and acceptability among HCWs. HCWs believe that 2wT will improve on-time ART visit attendance and provide valuable early retention in care support. The co-creation approach appears successful in designing an app that will meet HCW needs and, therefore, support client adherence to visits.