public/scripts/extensions/expressions/add-custom-expression.html

<h3>
    Enter a name for the custom expression:
</h3>
<h4>
    Requirements:
</h4>
<ol class="justifyLeft">
    <li>
        The name must be unique and not already in use by the default expression.
    </li>
    <li>
        The name must contain only letters, numbers, dashes and underscores. Don't include any file extensions.
    </li>
</ol>