Here is an option, first the HTML:
<select id="fnivel">
<option value="1">one</option>
<option value="2">two</option>
<option value="3">three</option>
</select>
<input id="fnivel2" hidden="hidden" />
The JavaScript:
$("#fnivel").change(function () {
var selected_option = $('#fnivel').val();
if (selected_option === '2') {
$('#fnivel2').attr('pk','1').show();
}
if (selected_option != '2') {
$("#fnivel2").removeAttr('pk').hide();
}
})
Here's a JsFiddle example.
Another approach, depending on your requirements is to create the input control programmatically, e.g.
$('#fnivel2').append('<input id="fnivel2" pk="1"/>').show();