O365: Skype for business 2016 - GPO for switching off splash screen

When you need to disable splash screen for users you, can apply group policy with preferences/register settings adding this key:
HKCU\Software\Policies\Microsoft\Office\16.0\Lync\DisableSplashScreen
with value of 1.
Additionally, you need to have update installed:
https://support.microsoft.com/en-us/help/3128049/january-3--2017--update-for-skype-for-business-2016-kb3128049

Source:
https://support.microsoft.com/en-us/help/3213034/option-to-suppress-the-skype-for-business-2016-startup-splash-screen