Launch Skype for Business.
Click the gear icon (⚙) in the top-right corner
In the Options window, select Personal from the left-hand menu.
Under the My account section, uncheck:
Automatically start the app when I log on to Windows
Start the app in the foreground
Click OK to save your changes, then close Skype for Business.
Click the Windows Start menu and type Startup Apps in the search bar and Select the one showing as in screen shot below.
Find Skype for Business in the list of apps and toggle the switch to Off.