Validator: The value must be equal to the desired value

How can I check if the value passed is equals to my desired value in the validation phase.

$rules = [
            'email' => 'required|email|max:255|unique:users',
            'password' => 'required|min:6',
            'admin_code' => 'required|<the value must be equals to this code "admin">',
        ];

via Chebli Mohamed