I have only one department in the template group. So what's the point of selecting a department then? Here is the solution:
Open Admin CP->Templates->Manage Groups
Select the Template Group you want to edit. (click on the name)
Go to the General->Index template Code:
<{foreach key=key value=item from=$widgets}>
<{if $item[split] eq 0}><tr><{/if}>
<!-- BEGIN WIDGET: <{$item[title]}> -->
<a href="<{$item[link]}><{if $item[link] == "index.php?_m=tickets&_a=submit"}>&departmentid=<{ $tgroup[departmentid]}>&step=1<{/if}>" id="moduletitle"><strong><{$item[title]}></strong></a>
<!-- END WIDGET: <{$item[title]}> -->
<{/foreach}>