How to setup MAME and Livesplit to work together?

One video instruct to click inside LiveSplit & WSPLIT- start the timer go back to your game and start the game.

But this will not provide the exact start time o navigate between a game and the speedrun timer via mouse clicks to start and stop.

How does other games "kick start" the start of the game inside the game software and automatically starts the timer at the same time?

I did set in LiveSplit (Global hotKeys) the start key to spacebar and also set the start key in MAME to spacebar.

Anyone that did got MAME and a speedrun timer to work automatically so that when "START"is press inside MAME then also the timer starts?