There's usually a retaining plastic that hold the primer inside the tank. Using a needle nose or a small cutter (be creative), you want to cut the primer from the inside of the tank. If you can't do this due to the design of the tank, you may pry it off but be really careful and try not to crack the tank.
Once the primer is off, find a screw slightly larder than the hole from the primer and plug it in.
The primer is something that you will see with RTR cars but they are not really needed. All you need to do is cover the exhaust (use a rag if it's hot) and pull the pullstart (or bump with a starter box) to prime the motor.