API call to have a user watch specific category(ies)?

Have you figured out how to achieve this?

Here is how to do it using jQuery Ajax for anyone interested:

const data = {
	notification_level: 3, 
	api_key: "all_users_api_key",
	api_username: "target_user_username"
};

const settings = {
  url: "/category/{{category_id}}/notifications",
  method: "POST",
  data: data
};

$.ajax(settings).done(response => {
  console.log(response);
});