$('select[id=currency-data]').change(function () {

    var currency_id =   $(this).val();

    $.ajax({
        url: "{{Route('exchange-rate')}}",
        type: 'POST',
        data: {currency_id: currency_id },
        success: function(data){
            $('#ex-rate').val(data);
        }
    });
});

{!! Form::label('currency_id', 'Currency:', ['class' => 'control-label']) !!}
        {!! Form::Select('currency_id', $currency_data, Input::old('currency_id'), ['id'=>'currency-data','class' => 'form-control','required']) !!}

        {!! Form::label('exchange_rate', 'Exchange Rate:', ['class' => 'control-label']) !!}
        {!! Form::input('number','exchange_rate', Input::old('exchange_rate'), ['id'=>'ex-rate','class' => 'form-control','readonly','required']) !!}