Check box not working on form before allowed to submit

Hello guys trying to get my checkbox to need to be clicked before submitting my button which is an onclick not submit??

<form>
  <p>
    <input style="padding:14px; -webkit-border-radius: 30px; -moz-border-radius: 30px; border-radius: 30px; width: 300px; border: none;" placeholder="Enter Date Here... e.g 17/05/1981" />
  </p>

  <p>

    <input type="checkbox" id="vehicle3" name="vehicle3" value="Boat" required="true"> <label style="color: #fff; font-size: 10px;"> Please Accept Terms</label></p>

  <p><a id="generateButton" href="generate.html" class="progress-button red" data-loading="Creating..." data-finished="Start Over" data-type="background-vertical" onclick="getRandomImage()">Start Search</a></p>
</form>