Assign variable to non existing transform

Hello, I have found a script witch makes it possible to enter and exit a car. For the script to work you need to assign a number of variables: vehicle, player, exitPoint, doorTriggerLeft, carCamera and playerCamera. The problem is that I spawn the player from a prefab if he joins a server (it is a multiplayer game) or if he starts a server. But I can't assign a transform that is not yet in the scene to a variable. I have tried to make a Global that is true if the player spawns and as soon as the player comes into the scene I set the player variable to the player transform like so

 if(PlayerCamera.GetComponent("MPBase").ingameGlobal == true)

     player = transform.Find("test22 (clone)");

but that doesn't seem to do anything. MPBase is my multiplayer script, ingameGlobal is the variable that sais wheter there is a player ingame or not and test22 (clone) is the name of the transform after initialising.

Does anyone have an idea about how to het this working I would greatly appreciate help.

(if you think my English is bad you're right, i'm dutch and stil in school)