pdb-mc-custom-tag.php

<?php

/**
 * Plugin Name: PDb MailChimp Custom Merge Tags
 * Description: adds a custom merge tag to the MailChimp registration
 * Author: Roland Barker, xnau webdesign
 * Version: 1.0
 * Author URI: https://xnau.com
 * License: GPL3
 */

class PDb_MailChimp_Custom {
  
  /**
   * sets up the filters
   */
  public function __construct()
  {
    // this filter gives you access to the data before it is sent to MailChimp
    add_filter( 'pdbmc-data_before_send', [$this,'add_merge_tag'] );
  }
  
  /**
   * adds the merge tag to the MailChimp registration data
   * 
   * @param array $mc_data
   * @return array
   */
  public function add_merge_tag( $mc_data )
  {
    // the "full_address" field is not a real database field, just a name for the extra data we're adding
    // "full_address" is the local name for the field, "M_ADDRESS" is the MC merge tag name for the field
    $mc_data['full_address'] = $mc_data['address'] . ' ' . $mc_data['city'] . ' ' . $mc_data['state'] . ' ' . $mc_data['zip'];
    
    return $mc_data;
  }
  
}

new PDb_MailChimp_Custom();

You must configure the merge tag you are going to use for your custom tag, both in the Participants Database MailChimp Integration plugin and in your MailChimp account audience settings.

You can ignore the warning about the field not being a Participants Database field if you see it.