Add a input after the select and make it as display none
<label for="city">City</label>
<select id="city" name="city" required>
<option value="">Select city</option>
<option value="city1">city1</option>
<option value="city2">city2</option>
<option value="other" onselect="">Other...</option>
</select>
<input id="other" style="display:none" />
and inside the script tag
<script type="text/javascript">
$(document).ready(function () {
$('#city').on('change', function (e) {
var selectValue = this.value;
if (selectValue == "other") {
$("#other").show();
}
else {
$("#other").hide();
}
});
});
</script>
and you also should add Jquery in your html file before the above script tag
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.0/jquery.min.js"></script>
after combine the Html and Scripts your html file should look like this
<html>
<body>
<label for="city">City</label>
<select id="city" name="city" required>
<option value="">Select city</option>
<option value="city1">city1</option>
<option value="city2">city2</option>
<option value="other" onselect="">Other...</option>
</select>
<input id="other" style="display:none" />
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.0/jquery.min.js"></script>
<script type="text/javascript">
$(document).ready(function () {
$('#city').on('change', function (e) {
var selectValue = this.value;
if (selectValue == "other") {
$("#other").show();
}
else {
$("#other").hide();
}
});
});
</script>
</body>
</html>