JSFiddle - React, Tailwind, and code Playground

HTML

<div class="width-qrtr">
   <label>Type of event</label>
  <select name="" id= "select">
      <option value="1">Select Type</option> 
      <option value="2">Golf Day</option>
      <option value="3">Dinner</option>
      <option value="4">Dinner and Golf Day</option>
      <option value="5">Other</option>
 </select>
  </div>
<div class="width-qrtr">
   <label>If you choose 'Other'</label>
    <input id="textbox" disabled="true" type="text"   value=""  name=""/>
</div>

JavaScript

document.getElementById('select').addEventListener('change', function() {
    if (this.value == 5) {
        document.getElementById('textbox').disabled = false;   
    } else {
        document.getElementById('textbox').disabled = true;
    }
});