Auto login with Chromium in kiosk mode for digital signage Options

[nsl]

I hope this is the right forum, otherwise please move. I've unexpectedly been tasked with setting this up for one of our customers, apparently because I do own a Raspi 3 running Raspbian

I've browsed various resources but I seem to be unable to get this to work. Chromium does start up in kiosk mode, loads the webpage and autofills the saved credentials but I still need to click on 'sign in'. I can't do that however, since there will be no keyboard/mouse attached when the system is eventually installed in the store (where it will be disconnected from power instead of being shut down properly every night).

Is there anyway I can do this with client-side scripting/userscripts?