I actually saw that thread when I wrote my example. But I used a different method as I was just capturing and not saving.
I wrote a post that explains how to control an iOS/MacOs app from Hype
The thread is long and evolved and there is more than one thing being shown.
But the gist of what you need to the is treat the Parent app the same way you would if you were to trying and control an iframe embedded in a Hype scene.
Post a message.
You can not control the app directly from a Hype scene. Even though the scene is being displayed within it.
What you can do is broadcast a message from the Hype scene (JS) and have the App listen out for a message coming from the Hype scene. ( actually WKWebViews content, which will be your scene )
If the message is of the correct name and type then the App can react to it using any function/code in the app you have written for taking a snapshot.
Messages can actually be sent from the App to the Hype Scene also.
And sorry before you ask I could post my example here but I honestly believe that you need to do some research so you gain some understanding. There are a couple of example already in the thread I listed, a little out of date due to being written in Objective - C and not swift if I remember correctly. But essentially the same methods used.