<form action="<?= $block->escapeUrl($block->getSaveFormAction()) ?>"
        id="newsletter-form"
        class="newsletter-form"
        method="post"
        data-hasrequired="<?php /* @escapeNotVerified */ echo __('* Required Fields') ?>"
        data-mage-init='{"validation":{}}'>
    ...
    <input type="text" name="newsletter[firstname]" placeholder="First Name" />
    <input type="text" name="newsletter[lastname]" placeholder="Last Name" />
    <input type="text" name="newsletter[email]" placeholder="Email Address" />
    ...
</form>

public function execute() {
    $post = (array) $this->getRequest()->getPost();
    // Get newsletter data
    $newsletterData = $post['newsletter'];
    $transport = $this->transportBuilder
            ->setTemplateIdentifier('newsletter_email_email_template')
            ->setTemplateOptions([
                    'area' => MagentoFrameworkAppArea::AREA_FRONTEND,
                    'store' => $storeId()
                ])
            ->setTemplateVars($newsletterData) // set newsletter data in email template
            ->setFrom(['email' => $fromEmail, 'name' => $fromName])
            ->addTo($data['email'])
            ->setReplyTo($fromEmail, $fromName)
            ->getTransport();
        $transport->sendMessage();
}

First Name: {{var firstname}}
Last Name: {{var lastname}}
Email Address: {{var email}}