To set up the official Autosplitter, first right click on your livesplit window and select "edit splits". After doing so, when you set your game to Thief (2014) you should see that under "Start timer at: " there is now a small description of the autosplitter and what it does, and to the side of it there should now be an "Activate" button to press. Just simply hit it, and you are good to go!

Alternatively, if you've already had your splits set up before the Autosplitter was added to the livesplit database, you should have a pop up window appear telling you of the Autosplitter the first time you open your old Thief (2014) splits, so just follow the prompt there. Please use this in runs going forward as it will make the leaderboards more accurate and ease workload on mods going forward. Thanks!

If this is somehow not working for you, you can also manually set up the Autosplitter by downloading the raw script file linked in resources - the one by Mysterion. That shouldn't be needed as the method above is quite simple and easy, but just in case :).