Platforms - defined as businesses with network effects, thereby becoming more valuable the more users they acquire - have become economically important in our current economies. They are ubiquitously present in our daily lives and drive an increasingly share of our economies. However, most platforms cease to exists within the first five years. Therefore, the goal of this study is to gain knowledge that can increase the chances of a platform reaching its critical mass – the minimum number of users required to be commercially viable. In this study, 66 healthcare providers, one industry collaborative, and one start-up participate in an action design research (ADR) to develop an entirely new digital platform within a commercial environment. The research - consisting of a knowledge-base review, interviews, focus-groups, a questionnaire - was split into four stages: 1) Problem Formulation, 2) Build Intervention and Evaluation Cycles, 3) Reflection and Learning, and 4) Formalization of Learnings. This study concluded with a failure of the platform to reach critical mass. The learnings produced by the instantiated platform and - problem are generalized, resulting in the articulation of three commercial platform design principles. These design principles aim to help the practitioner increase the chances for their platform to reach critical mass. The design principles being: Growth by Design, Mutation by Design, and Leadership by Design. Current theories were found to be primarily descriptive and suffer from a survival bias. This, as they are largely based on after-the-fact (ex-post) research and data. Furthermore, they do not account for irrational actor behavior or commercial implications. Therefore, further research is suggested, focused on the time between platform inception and reaching critical mass.