edit default automatically username after wordpress registration

Hi, I’m trying to change the username that is automatically generated when a user registers on my site, I would like the username to always be first_name.last_name, I’ve tried different hooks but unfortunately the result never changes, every time it is generated in automatic way a username that decides the system, how can I solve it? This is the last snippet I used:

add_action(‘um_registration_before_auto_user_id’, ‘custom_username_from_name_and_lastname_UM’, 10, 1);

function custom_username_from_name_and_lastname_UM($args) {
$first_name = isset($args[‘first_name’]) ? $args[‘first_name’] : ”;
$last_name = isset($args[‘last_name’]) ? $args[‘last_name’] : ”;

if ($first_name && $last_name) {
    $username = sanitize_user($first_name . '.' . $last_name);
    $args['user_login'] = $username;
}

return $args;

}

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