I just tested this out since I'm going to probably have to start using this at some point. According to the API documentation, the "template id" for now is just the word "default". Here's what the documentation says:

Using 'default' as the template_id should suffice for the current implmentation (as there should be only one template per course). However, using specific template ids may become necessary in the future

So in the Put command, you're deciding what courses to associate with a blueprint course. All the association stuff happens in the blueprint course so the "course_id" is the course id for the blueprint. The "courses ids to add" array lists all the courses you want associated with the blueprint.

For example, if the blueprint course is 1111 and you want to associate course 1112 then the url in Postman will look like this: