Go to your track of choice and do the following:
Press shift+u
Click edit at the bottom of the screen
Click control at the top right of your screen
Select autocross start
Turn heading until the arrow on track points to desired direction
Point the arrow where you want to place the starting position and press O
Click save layout in bottom right, name it anything (i name mine FE5_start and for some cars I might have closer or further track position depending on acceleration and cornering speed so I might have 3 different starts for different cars)
Press shift+r and your car will spawn where you put the arrow