Thanks a lot for your answer.
I was trying to insert the code you provide in auth.tpl but it doesn't work . I don't get anything in authentication box.
This is a portion of my auth.tpl where I am trying to get it working.
{else} {* use_secure_login_page *}
<tr><td class=VertMenuItems>
<font class=VertMenuItems>{$lng.lbl_username}</font>
<input type=text name=username size=16 value="{#default_login#}">
<font class=VertMenuItems>{$lng.lbl_password}</font>
<input type=password name=password size=16 value="{#default_password#}">
<input type=hidden name=mode value=login>
{if $active_modules.Simple_Mode ne "" and $usertype ne "C" and $usertype ne "B"}
<input type=hidden name=usertype value="P">
{else}
<input type=hidden name=usertype value="{$usertype}">
{/if}
<input type=hidden name=redirect value="{$redirect}"></td></tr><tr><td height=24 class=VertMenuItems>
{if $js_enabled}
{include file="buttons/login_menu.tpl"}
{else}