JSFiddle - React, Tailwind, and code Playground

HTML

<select id="jobtype[]" class="dropDown" size="4" multiple="multiple" name="jobtype[]">
    <option value="28">Activity staff/ Resort Rep</option>
    <option value="1">Admin/ Secretarial/ Clerical</option>
    <option value="2">Advertising/ Marketing/ PR</option>
    <option value="29">Art/ Design</option>
    <option value="39">Au Pair/ Nanny</option>
</select>

JavaScript

$("#jobtype\\[\\] option[value=39]").prop('selected', true);