How to send argument to drupal_get_form()
This is how you send arguments to your own form when using drupal_get_form() in Drupal 6.x. This function has no required argument, but receives $form_state as first argument. Declare a first argument for $form_state and then your arguments, as many as you need.
<?php
function my_module_begin_form($form_state, $my_arg = array(), $another = "") {
...
}
...
$out = drupal_get_form('my_module_begin_form', $my_arg, $another);
?>