To add a full-text search (title and messages, like in the "issue.search" template) to the sidebar, add the following to the page.html template in your tracker's "html" directory:

 <form method="GET" action="issue">
  <input type="hidden" name="@template" value="index">
  <input type="hidden" name="@columns"
         value="id,activity,title,creator,assignedto,status">
  <input type="hidden" name="@sort" value="activity">
  <input type="hidden" name="@group" value="priority">
  Search: <input name="@search_text" size="10">
 </form>

The mechanisms used here are described in the user guide and customisation guide