If the simple plan of realization came into his area... the finish line should be a Collider, wishes him a script with OnCollisionEnter method if it is for 3D,2D in the same way, however it's in the unity documentation.
If you need to go through the finish line, but it worked for the script. Put in the component of the Collider and tick IsTrigger same work, but with the method OnTriggerEnter
If you just need to check from above whether the player than the finish, then take the point of surface finish and even the odds with the position of the feet of the player, for this player you can add the game object to the area of the legs.
But I can guess that you just need to know the contact, but okay,let it be.