Not redirecting the subdomain using .htaccess
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I have site which uses wordpress and I usually login as : mydomain.com/wp-admin . I want it to be on the subdomain .
I have tried to create a subdomain by using the following rewrite url :

    RewriteCond %{HTTP_HOST} ^wp-admin
    RewriteCond %{REQUEST_URI} !^/wp-admin
    RewriteRule ^(.*)$ wp-admin/$1 [L]

But when I am trying to reach`http://wp-admin.mydomain.com/` a weired page comes out saying  an error message : 

`If you feel you have reached this page in error, please contact the web site owner:`
 
And the url changes to :

    http://wp-admin.mydomain.com/cgi-sys/defaultwebpage.cgi
Please help how could I be able to create a subdomain .