title: auto populate a text field on a dropdown selection in a tpl file
body: <p>I have a dropdown like this</p>
  
  <pre><code>   &lt;div  class="constraint constraint-pattern , panel" {{#unless 
                  constraints.pattern.selected}}style="display:none"{{/unless}} &gt;
          &lt;h3 class="full-width"&gt;{{__ "Select here"}}&lt;/h3&gt;
          &lt;select name="constraint" class="select2" data-has-search="false"&gt;
              &lt;option value="[0-2]"&gt;abc&lt;/option&gt;
              &lt;option value="[2-4]"&gt;bcd&lt;/option&gt;
              &lt;option value="[4-9]"&gt;bnm&lt;/option&gt;
          &lt;/select&gt;
      &lt;/div&gt;
  </code></pre>
  
  <p>I want to popultate a text area when selecting a value from the dropdown.</p>
  
  <pre><code>    &lt;div class="constraint constraint-pattern" {{#unless 
                  constraints.pattern.selected}}style="display:none"{{/unless}}&gt;
          &lt;label&gt;
              {{__ "Type"}}
          &lt;/label&gt;
  
          &lt;input type="text" readonly name="patternMask" value="abc"/&gt;
      &lt;/div&gt;
  </code></pre>