Welcome to Lenovo Community! As per the query we understood that you are facing looking to turn OFF built in Display on your ThinkPad S1 Yoga. You can use Windows key+P keys as shortcut key to switch the displays, please check with below link for further assistance ...
Lenovo Thinkpad T61 - I created a new Teensy 4.0 connector board for a T61 which will work on all T60 series keyboards. In addition to controlling the keyboard and trackpoint, the code also controls the touchpad. The code, circuit boards, and PDF are in a folderat my repo. See Step 17...