Custom slack_wpcf7_submit_message

functions.php

<?php

/* $message — Default message to send to Slack
   $form — Form object
   $result — Array of result 
   */
function custom_cf_7( $message, $form, $result ) {
    return "OK" . json_encode($form) . json_encode($result); // Customize message here
}
add_filter( 'slack_wpcf7_submit_message', 'custom_cf_7', 10, 3 );

/* Outputs this :
OK{}{"contact_form_id":6,"status":"mail_sent","message":"Thank you for your message. It has been sent.","demo_mode":false} 
*/