<section class="widget search clearfix">
    <form class="form-inline" role="form" method="post" action="">
        <div class="input-group input-group-lg">
            <input type="text" name="busca" required class="form-control lg" placeholder="Busca…">
            <span class="input-group-btn">
                <button type="submit" class="btn btn-sm">Buscar</button>
            </span>
        </div>
    </form>
</section>