<select name="ulke" onChange="document.getElementById('adres').value+=this.value;">
<option>Ülke Seçiniz</option>
<option value="Türkiye ">Türkiye</option>
</select>
<select name="sehir" onChange="document.getElementById('adres').value+=this.value;">
<option>Şehir Seçiniz</option>
<option value="Ankara">Ankara</option>
</select>
<input type="text" name="adres" id="adres" size="30">