<form action="index.php?menu=blog&section=admin" method="post">

RewriteCond %{HTTP_HOST} ^example.com$
RewriteRule ^(.*) http://www.example.com/$1 [QSA,L]

#RewriteRule ^(.*) http://www.example.com/$1 [QSA,L,R=301]
#RewriteRule (.*) http://www.example.com/$1 [QSA,L,R=301]
#RewriteRule ^(.*) www.example.com/$1 [QSA,L,R=301]
#RewriteRule ^(.*) http://www.example.com/$1 [QSA,NC,P] #Doesn't rewrite "www"