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Here are 3 ways to do it

1. Use the wp_signon hook

   $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)