How to use Laravel Accessor in Laravel Validation

Question

I want to use laravel accessor in validation but i didn't figure the right way to do this my model (one currency for each country)

class Currency extends Model
{
    protected $table = 'currency';

    protected $fillable = [
        'code', 'country_id'
    ];

    public function country()
    {
        return $this->belongsTo(Country::class);
    }

    public function getCountryCodeAttribute()
    {
        return $this->country->alpha_2;
    }
}

example of the form

<input type="text" name="code">
<select name="country">
    <option value="" disabled selected>Choose your option</option>
    <option value="FR">France</option>
    <option value="PT">Portugal</option>
    <option value="ES">Spain</option>
</select>

In my controller i tried this

public function addCurrency(Request $request){
    
    $validator = Validator::make($request->all() , [
        'code'      => 'required|string' ,
        'country'   => ['required', 'exists:country,alpha_2', Rule::unique('currency', 'country.alpha_2') ],
    ]);

    if ($validator->fails()) {
        return response()->json([
            'errors' => $validator->errors(),
        ]) ;
    }
}

and also this

public function addCurrency(Request $request){
    
    $validator = Validator::make($request->all() , [
        'code'      => 'required|string' ,
        'country'   => ['required', 'exists:country,alpha_2', Rule::unique('currency', 'country_code') ],
    ]);

    if ($validator->fails()) {
        return response()->json([
            'errors' => $validator->errors(),
        ]) ;
    }
}

but didn't work

PS: i don't want to use the alpha_2 as the foreign key instead of country_id and i don't want to use country_id in the value options in form