functions.php

<?php

/**
 * Limit ConvertKit signup based on checkbox
 *
 */
function ea_wpforms_limit_convertkit_signup( $process, $fields, $form_data ) {

	// Limit to user registration form
	if( 20 != $form_data['id'] )
		return $process;

	// Don't process if checkbox is empty
	if( empty( $fields[5]['value_raw'] ) )
		$process = false;

	return $process;
}
add_filter( 'be_convertkit_process_form', 'ea_wpforms_limit_convertkit_signup', 10, 3 );