This codes makes the Mailchimp checkbox to be required by default. The user will not be able to donate without checking the newsletter checkbox.

required-givewp-mailchimp.php

<?php 
function required_givewp_mailchimp() { ?>
	<script>
        jQuery("input[name=give_mailchimp_signup]").prop("required", true);

	</script>

<?php }

add_action( 'give_donation_form_after_submit', 'required_givewp_mailchimp' );