How set default value as full url and make filed disabled in payment config page

[vishwanath]

Hi All,

I have make config.xml file and have one field as notification url
<item id="custom_hpp_notificationURL">
            <type>input</type>
            <default_value>HTTP_SERVER/index.php?rt=extension/custom_hpp/callback</default_value>
        </item>

i want two things here :

1) it will return full URL in admin on payment configuration page as
fullURL/index.php?rt=extension/custom_hpp/callback

2) make this filed disabled or non editable for admin.

How can i do this? Please help me.
Thnaks,