Intercept and modify the payload sent by Advanced Forms

customise-advanced-forms-mail-payload.php

<?php

// Set the form key you wish to target  
$form_key = 'form_5d97cf9edc0a8';

add_action( "af/email/before_send/key=$form_key", function ( $email, $form ) {

	add_filter( 'wp_mail', function ( $data ) use ( $email ) {

		// you can override any items in this array to customise the email that is sent...
		// $data['to'];
		// $data['subject'];
		// $data['message'];
		// $data['headers'];
		// $data['attachments'];
    
    // e.g;
    $data['subject'] = 'This is a custom subject';

		return $data;

	} );

}, 10, 2 );