JSFiddle - React, Tailwind, and code Playground

HTML

<select name="theme" onchange="changeUrl()" id="dropdown">
  <option value="">Please select a theme...</option>
  <option value="Standard">Standard</option>
  <option value="Google Adwords">Google Adwords</option>
</select> 
      
<a href="url.php?mode=modename&theme=xxx" target="_blank" id="link">Preview</a>

JavaScript

function changeUrl()
{
  var dropdown = document.getElementById("dropdown");
  var link = document.getElementById("link");
  if(dropdown.value == "Standard")
  {
    link.href = "url.php?mode=modename&theme=Standard";
  }
}