JSFiddle - React, Tailwind, and code Playground

HTML

<fieldset class="col-12-box-bottom add-extras">
    <legend class="plus">Add Promotion Code</legend>
    <ul id="promo-fields">
      <li><input class="field-small" type="text" /></li> 
      <li><button class="but-sec" type="submit">Apply</button></li>
    </ul>
  </fieldset>

CSS

.add-extras legend{
    width: 260px;
    border: 1px solid red;
    display: block;
}

legend + * {
  -webkit-margin-top-collapse: separate;
  margin-top: 10px;  
}