<div class="d-flex flex-column flex-sm-row form-group">
    [text* email class:form-control class:form-control-lg class:mr-sm-2 class:mb-2 class:mb-sm-0 class:h-100 placeholder "Email Address"][submit class:btn class:btn-lg class:btn-primary class:ml-sm-2 "Subscribe"]
</div>