Te comparto el código para tu Webhook

<?php

// Automatiza tu negocio con Herramientas NoCode o LowCode  delete.php?secret=CLAVE

// ActiveCampaign url & API Key
$url = '';
$api_key = '';
$secret_key = 'CLAVE';  // palabra clave para borrar contactos a través de Webhook sólo letras y números


/***** do NOT edit below this line *****/

/*****
* functions
*
*****/
function go_curl($id, $api_action)
{
    global $url, $api_key;
    
    if ($api_action == "contact_delete")
    {
        $params = array(
            'api_key'                   => $api_key,
            'api_action'                => $api_action,
            'api_output'                => 'serialize',
            'id'                        => $id,
        );
    }
        
    $query = "";
    if (!empty($params)) foreach( $params as $key => $value ) $query .= urlencode($key) . '=' . urlencode($value) . '&';
    $query = rtrim($query, '& ');
    
    $data = "";
    if (!empty($post)) foreach( $post as $key => $value ) $data .= urlencode($key) . '=' . urlencode($value) . '&';
    $data = rtrim($data, '& ');

    $url = rtrim($url, '/ ');
    
    if ( !function_exists('curl_init') ) die('CURL not supported.');
    
    if ( $params['api_output'] == 'json' && !function_exists('json_decode') ) {
        die('JSON not supported. (introduced in PHP 5.2.0)');
    }
    
    $api = $url . '/admin/api.php?' . $query;
    
    $request = curl_init($api);
    curl_setopt($request, CURLOPT_HEADER, 0);
    curl_setopt($request, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($request, CURLOPT_POSTFIELDS, $data);
    curl_setopt($request, CURLOPT_FOLLOWLOCATION, true);
    
    $response = (string)curl_exec($request);
   
    curl_close($request);
    
    if ( !$response ) {
        die('Nothing was returned. Do you have a connection to Email Marketing server?');
    }    
    
    return $response;
}

$id = trim(strip_tags($_POST['contact']['id']));
$secret = trim(strip_tags($_GET['secret']));
$api_action = 'contact_delete';

if ($id > 0 && $secret === $secret_key && $secret_key != "") $result = unserialize(go_curl($id, $api_action));

Recursos del vídeo