How to add event to Teams global Calendar on behalf of a channel instead of a team group?

libertitas the technical answer is that it's not possible because a channel isn't a mail object like the Team is or users are.

A broader answer is that there are lots of ways that Teams doesn't interact well with calendars due to the historic way in which exchange is structured where everything needs to be owned by a single mail object. It'll make a lot of changes to how calendaring works entirely to change this.