Publisher subscriber pattern communication between Angular 6 applications via iframe

I'm working on a solution where an angular application is consumed inside an iframe of another angular application. The child app need some parameters to show the correct information into the app parent. I read a lot of solution around the web and a good solution is to apply the design pattern publisher/subscriber.

A lot of example are between component inside the same application and I'm wondering if the same solution with rxjs objects is the right way (as shown here).

My doubt is: how I subscribe the child application to the parent (an external publisher)?

I was thinking on a REST API exposed by the parent for registration and unregistration of subscriber, but I'm not sure.

Any experience with this situation? Thanks in advance