Here are 3 ways to do it

 1. Use the [wp_signon hook][1]

   `$creds = array();    
    $creds['user_login'] = 'example';   
    $creds['user_password'] = 'plaintextpw';   
    $creds['remember'] = true;   
    $user = wp_signon( $creds, false );  
    if ( is_wp_error($user) )  
       echo $user->get_error_message();`

 

 2. Use jQuery Val and add it to your login page

   `$("#user_login").val('username');    
     $("#user_pass").val('password');`

 3. Modify wp-login.php (Note, this is not recommended although it would work)



  [1]: http://codex.wordpress.org/Function_Reference/wp_signon