I have tried to follow what is being said above but it is not clean cut.
I have a subscription form that is called Subscription form. It has three fields seen in the attachment

I have created the following PHP script. When I test it from the front end it says successful however I do not see any record created in the backend on Acymailing.
// get form data
$postData = $_REQUEST['Subscription Form'];
// check if user wants to subscribe
if(empty($postData['Submit'])) return; { // change value between brackets and single quotes to your own fieldname
include_once(rtrim(JPATH_ADMINISTRATOR, DIRECTORY_SEPARATOR).DIRECTORY_SEPARATOR.'components'.DIRECTORY_SEPARATOR.'com_acym'.DIRECTORY_SEPARATOR.'helpers'.DIRECTORY_SEPARATOR.'helper.php');
$userClass = acym_get('class.user');
$myUser = $userClass->getOneByEmail(strip_tags($postData['Email'])); // change value between brackets and single quotes to your email address fieldname
// check if email address already exists
if (empty($myUser)) {
$myUser = new stdClass();
}
$myUser->email = strip_tags($postData['Email']); // change value between brackets and single quotes to your email address fieldname
$myUser->name = strip_tags($postData['First Name']); // change value between brackets and single quotes to your name of the person fieldname current field value will always be updated with form value
$myUser->name = strip_tags($postData['Last Name']); // change value between brackets and single quotes to your name of the person fieldname current field value will always be updated with form value
// Custom fields
$customFields = [];
$customFields['2'] = strip_tags($postData['Email']); // change value between brackets and single quotes to your own fieldname current field value will always be updated with form value
$customFields['1'] = strip_tags($postData['First Name']); // change value between brackets and single quotes to your own fieldname current field value will always be updated with form value
$customFields['3'] = strip_tags($postData['Last Name']); // change value between brackets and single quotes to your own fieldname current field value will always be updated with form value
$myUser->id = $userClass->save($myUser, $customFields);
$listToSubscribe = [3]; // change value between brackets to your id of the acymailing list use comma for multiple lists
$userClass->subscribe($myUser->id, $listToSubscribe);
}
Here is a view of my Acymail custom fields

What is wrong here?
And can you please update your documentation to include version 6 requirements.