<form id="form">
<input type="text" value="example.ru" readonly="" name="domain" style="text-align:right" /> /
<input type="text" value="" name="path" /> <button type="button" name="button">Открыть</button>
<script>
var form = document.getElementById("form");
form.button.onclick = function() {
var url = "http://" + form.domain.value + "/" + form.path.value;
window.open(url);
}
</script>
</form>