Registering the provider

As shown in section Screenshots, you have the opportunity to register your provider and add it to the selector of registered providers:

Sample code to be added to your ext_tables.php file:

$GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['direct_mail_userfunc']['userFunc'][] = [
    'class'  => 'yourClass',
    'method' => 'yourMethod',
    'label'  => 'Some Label'
];

This will add a new entry "Some Label" that fills in provider yourClass->yourMethod when selected: