Hi - So I've come across a solution for this, and it's currently working for me using Unity 2017.1.1f1 - I've seen the question pop up a lot and gone unanswered on the web -
I used the method posted here:
https://forum.unity.com/threads/how-to-screenshot-in-a-new-tab-of-the-browser-without-pop-up-blocking.440051/However, I skipped the step where you make a button, and also skipped the "PressHandler" script that you attach to that button..
Instead, I used the "Invoke Method" action in playmaker. this action could be triggered however you'd like using playmaker, in my case I chose to use "GetKeyDown" and used the "P" Key.
Hope this can help someone, thank you to Daniel and Valentin who put together the scripts