JSFiddle - React, Tailwind, and code Playground

HTML

<select class='page-dropdown' name='page_id' id='page_id'>
<option value="">Select... </option>
<option class="level-0" value="206">Subpage 1</option>
<option class="level-0" value="207">Subpage 2</option>
</select><br>
<select class='page-dropdown' name='page_id2' id='page_id2'>
	<option value="">Select... </option>
	<option class="level-0" value="208">Subpage 1</option>
	<option class="level-0" value="209">Subpage 2</option>
</select><br>
<select class='page-dropdown' name='page_id3' id='page_id3'>
	<option value="">Select... </option>
	<option class="level-0" value="210">Subpage 1</option>
	<option class="level-0" value="211">Subpage 2</option>
</select>

JavaScript

var dropdown = document.getElementsByClassName("page-dropdown");
function onCatChange()
{
    if ( this.options[this.selectedIndex].value > 0 )
    {
        console.log("http://www.example.com/?page_id="+this.options[this.selectedIndex].value);
    }
}
for (var i = 0, l = dropdown.length; i < l; i++) {
    dropdown[i].onchange = onCatChange;
}