WordPress MU

When trying to add custon input html tag like
<input type="text" name="s" />
<input type="submit" class="button" value="Search" />

(default WP search box) in a static page, those fields are stripped from the html code.

What I am actually missing? is this the correct behaviour? If yes, how can i change this, so that if I use input html tags when writing raw html code these are kept?

Thank you.

i'm having the same issue with a submit form. input tags are stripped.

WPMU uses the kses php html filters to provide security for blog hosters from having malicious content added to blogs, prevent cross site scripting attacks, etc.
http://sourceforge.net/projects/kses/

The forum here is filled with answers for working around it if you really need to.
http://mu.wordpress.org/forums/tags/kses