Hi,
guess the easiest way is to just pass some get parameters and use that as value for the topic.
So open "open.php" and add something like
$source = $_GET;
Now open "/include/client/open.inc.php", goto the opening of the actual form (somewhere around line 20) and add
<input type="hidden" name="topicId" value="<?php echo $source?>">
Now remove the table row, where the help topic dropdown is displayed.
Finally change the links from your Tech Support or Billing page to something like
<a href="/osticket/open.php?source=2">Create a Billing Ticket here</a>
or
<a href="/osticket/open.php?source=1">Create a Support Ticket here</a>
You have to lookup the the numbers for the "source" parameter for yourself, it's the value of the column "topic_id" in the table "ost_help_topic" for the respective topic.
Should work, haven't tested it for myself, but there shouldn't be too much problems with that. It would probably be a good idea to implement a check in open.php if there is a value for "status" and if not, set it to a default value (probably for support) to prevent having no topic at all.
Best
T.