My experience, having worked with Canvas, is that when the app is first defined according to these instruction, then whatever you enter in the "Launch url" slot will be entered anytime an instructor sets up an assignment as described here. Because the app is generally used for more than one webwork course, we only enter https://server.domain/webwork2/ as the launch domain. Then each instructor only needs to append to this url the "coursename/setname" or "coursename," depending whether they use the "homework" mode or the "course" mode, respectively.
The point of the "Launch URL" is to save each instructor from having to write the full webwork server URL every time they set up an assignment.
I don't use http or https in the domain. just server.domain.edu
Anyway, as posted in the last paragraph here, we need a better setup for instructors.