The category slug? Can you please explain a little more about this?
I’m assuming by category you mean the forum that the topic is in?
You can remove the forum dropdown menu and use a hidden field here instead.
In your theme you need to have this file:
wp-content/your-theme/bbpress/form-topic.php
If you don’t have it, copy it over from the bbpress default templates at:
wp-content/plugins/bbpress/templates/default/bbpress
Then open your new form-topic.php in a text editor and find this part of the code:
<p>
<label for="bbp_forum_id"><?php _e( 'Forum:', 'bbpress' ); ?></label><br />
<?php
bbp_dropdown( array(
'show_none' => __( '(No Forum)', 'bbpress' ),
'selected' => bbp_get_form_topic_forum()
) );
?>
</p>
and replace it with this:
<input id="bbp_forum_id" type="hidden" name="bbp_forum_id" value="20">
Where 20 is the ID no. of the forum you want all topics to be created in.
Hope that helps 🙂