I just started a while ago and i don't know how people get the timer to start automatically when they touch the yellow speed line in speedrun 4 on roblox. I have to switch windows to start the timer which makes my time slightly slower. Can someone explain how to get it to start and end automatically?

Game-related questions should be looked for or asked in the game forums, not here on the general forums. That also includes questions about autosplitters.

By the way, are you sure such thing exists? Quick search on the "Roblox: Speed Run 4" forums give this result: https://www.speedrun.com/robloxsr4/thread/nbbkg