This case study investigates how a designed learning environment cultivates a "sense of promisingness"--an ability to discern what may work in uncertain conditions, essential for creative expertise. The study focused on 32 in-service teachers enrolled in a Master's program in Taiwan, who engaged in mutually-supportive development of their thesis proposals over 18 weeks using the Knowledge Forum (KF) online tool. To evaluate the development of a sense of promisingness, we analyzed online discussion content where participants expressed their understanding of "what may work, how, and why" for improving their thesis proposals. We particularly examined three types of discourse moves--sharing-, argument-, and integration-oriented--to identify conditions fostering this ability. Key findings revealed that (1) sharing-oriented discourse was necessary but not sufficient for promoting the sense of promisingness, and (2) increased effort in integration-oriented discourse moves correlated with a higher likelihood of developing this ability. The study demonstrates that the course activity, designed based on knowledge-building principles in KF, serves as an effective instructional intervention for cultivating a sense of promisingness among graduate students. This research addresses a gap in the literature regarding methods to develop such knowledge-creating sense and the role of different discourse moves in sustaining knowledge advancement in online learning environments. The paper concludes with design implications for similar educational contexts.