By default, Canvas will show names from the Haven't submitted yet category.
In the drop-down menu [1], select the category of students you want to message. Based on real-time data, Canvas will show the names of the students who fall in the category you selected [2]. You can also remove students from the message by clicking the Remove icon.
Canvas will also generate a subject line based on the category [3], but you can edit the subject line if needed.