leaky-paywall-custom-field-to-active-campaign.php

<?php

add_action('leaky_paywall_active_campaign_after_new_subscriber', 'zeen_add_custom_field_for_active_campaign', 10, 2 );

function zeen_add_custom_field_for_active_campaign( $user_id, $contact_id ) {

    $field_id = 1; // retrieved from the admin settings custom fields table
    $value = sanitize_text_field( $_POST['2_field_8_1'] ); // custom registration field name

    $api = new Leaky_Paywall_Active_Campaign_Api();
    $api->add_custom_field($contact_id, $field_id, $value);

    // add additional custom fields here using the same process

}