how to make a screen grab funcitionality and assign it to a button?

Hi Per!

I think you want to capture the contents of your phone or tablet's screen during ThingWorx View session?

May be better and simple to use "native" phone (smartphone, tablet) function to snap screen picture.

You use iPhone or Android device?

iPhone:

Method One: Pressing the Home (1) and Sleep/Wake Button (2) to Snap

https://cdn.instructables.com/F9I/JPHK/I0H8ZR7U/F9IJPHKI0H8ZR7U.MEDIUM.jpg

"Simply (simultaneously) press and hold the “Sleep/Wake” button and then press the “Home” button. Once it’s done, you will hear a camera shutter which signifies that a screenshot is successfully taken." (Official instruction for iPHONEs

Method Two: google, please, "How to take a screenshot with the iPhone 6, iPhone 6 Plus, iPhone 7 ..." and select more suitable for snapshootnhg in your case

Concerning Android smartphones: you can use the similar method: to use main functionality to make shap:

Taking a screenshot on Android is usually as simple as simultaneously holding down the power button and volume-down button, but various alternative methods can be found on the great many Android phones and tablets on the market.

For a quick standard screenshot, first try exactly this: press the power button and volume-down button together, and hold for a few seconds until the screen flashes to signal the screengrab has been captured.

Regarding my experienses: I use iPAD and method one  (as described above) and made dozens of snaps for demonstration AR cases:

example 1 - prepare the AR model with widgetes and labels:

Then use it in the real device and fix AR presence in iPAD as a snapshots according method one above:

(You can see all three pictures are snaps from iPAD screen in ThingWorx View)

Hope I help you.