[destus]

MichaelDiteeeeeeee,

<script>
         document.addEventListener('DOMContentLoaded', function (e) {
             var selFrom = document.querySelector('select[name=from]'), selTo = document.querySelector('select[name=to]'), arr = [], pattern = /airport/i;
             selTo.disabled = true;
             selFrom.addEventListener('change', function (e) {
                selTo.innerHTML = selFrom.innerHTML;
                pattern.test(this.value) ? arr = [].filter.call(document.querySelector('select[name=to]'), function (el) {
                    return !(pattern.test(el.value));
                }) : arr = [].filter.call(document.querySelector('select[name=to]'), function (el) {
                    return pattern.test(el.value);
                })
                    selTo.options.length = 0;
                    [].forEach.call(arr, function (el) {
                        selTo.options.add(el);
                    });
                   selTo.disabled = false;
            })
        })
     </script>
     <select name="from" required>

        <option selected="true" style="display:none;"> </option>

        <option>- Suvarnabhumi International Airport</option>
        <option>- Don Mueang International Airport</option>
        <option>-  Bangkok City</option>
        <option>- Hua Hin</option>
        <option>- Cha Am</option>
    </select>
    <br>

    <h2>Drop Off Location </h2>
    <select name="to" required>

        <option selected="true" style="display:none;"> </option>
        <option>- Suvarnabhumi International Airport</option>
        <option>- Don Mueang International Airport</option>
        <option>-  Bangkok City</option>
        <option>- Hua Hin</option>
        <option>- Cha Am</option>
    </select>