After you insert the new user you need to get the value you want from the response and update the acf field.
You need to use the Field Key because the value will not already exist
$new_user_id = wp_insert_user($userdata);
// get value from response you want to set
update_field('field_XXXXXXXX', $value, 'user_'.$new_user_id);
https://www.advancedcustomfields.com/resources/update_field/
