How to check if string entered already exists in C

I am here with a doubt to create a function to check if a string entered by the user already exists in a struct.

I have the following struct to store cars data:

typedef struct
{
    char brand;
    char car_plate;
    char model;
} stCar;

For example when asked to enter the car_plate, i need a function to check if that car_plate already exists in the stCar struct because can not exist two cars with the same car_plate.

Someone can please help me?

Thanks.