silverstripe-webpack/app/templates/SilverStripe/UserForms/Control/UserDefinedFormController_ReceivedFormSubmission.ss

<div id="uff" class="userdefinedform__thx">
	<div class="on-complete-message user-form">
        $OnCompleteMessage
    </div>

    <% if $CustomThankYouCode %>
        <div class="custom-code user-form">
            $CustomThankYouCode.RAW
        </div>
    <% end_if %>
</div>