Cập nhật ngày 14/05/2022

HTML

<select id="selectbox" name="" onchange="javascript:location.href = this.value;">
    <option value="https://www.yahoo.com/" selected>Option1</option>
    <option value="https://www.google.co.in/">Option2</option>
    <option value="https://www.gmail.com/">Option3</option>
</select>

Javascript

<script type="text/javascript">
    window.onload = function(){
        location.href=document.getElementById("selectbox").value;
    }       
</script>

Jquery

jQuery(function () {
    // remove the below comment in case you need chnage on document ready
    // location.href=jQuery("#selectbox").val(); 
    jQuery("#selectbox").change(function () {
        location.href = jQuery(this).val();
    })
})