<form class="my-form omnisend-subscribe-form" action="/subscribe" method="post">
    <input class="omnisend-subscribe-input-email" type="text" placeholder="Enter your email address" />
    <input class="omnisend-subscribe-input-first-name" type="text" placeholder="Enter your first name" />
    <input class="omnisend-subscribe-input-last-name" type="text" placeholder="Enter your last name" />
    <input type="submit" value="Subscribe!">
</form>