This study explores the impact of mandatory participation in online discussion forums on learners' voluntary participation and learning performance in MOOCs. Analyzing a dataset of 27,767 learners from a Chinese MOOC platform, the research finds that mandatory participation not only directly improves learning performance but also significantly boosts voluntary participation. The effect varies, being stronger among early registrants and part-time learners. This work introduces mandatory participation as an effective strategy to enhance learner engagement and outcomes in online courses.