Oh very cool. This was known a year before I posed the question. I wonder if there has been anything since 2018?

This depends on exactly how your course defines min-cost flow (there are a few alternatives). If you go with the wikipedia definition at en.wikipedia.org/wiki/Minimum-cost_flow_problem then you have figured out the c's (capacity) and a's (cost) but you haven't defined d (the required flow). Other definitions have a supply/demand at every node. So re-lookup the definition in your course, and see which parameters you have not yet set.

And is there any documentation on the quality of the intlinprog of the Toolbox?