Mailchimp additional opt-in list check by default.

pmpro_mc_optin_checked_default.php

<?php

/**
 * This snippet sets the additional opt-in lists for MailChimp to be selected on Paid Memberships Pro checkout page.
 * Add this code below to your PMPro Customizations Plugin - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
 
 function pmpro_mailchimp_check_by_default() {
	global $pmpro_pages;

	//only load this on the PMPro checkout page.
	if( is_page( $pmpro_pages['checkout'] ) ) {
	?>
		<script>
			jQuery(document).ready(function() {
				jQuery('#additional_lists_1').prop('checked', true); //copy this and change the #additional_lists_1 to match the input checkbox ID's
			});
		</script>
	<?php
	}
}

add_action( 'wp_footer', 'pmpro_mailchimp_check_by_default' );