If your site is configured to view unassigned games, then you can use assignr.com to request a specific assignment from your assignor.
To request a game:
Click the Games menu item
Click the All Unassigned Games menu item
Find the game you want to request, and
Click Request It
Once you have requested a game, the screen will indicate that your request has been recorded, and your assignor will be notified via email of your game request. If you change your mind, and decide that you do not want the requested game, just click the Withdraw Request link.
If your request is granted, you will receive a game notification via email (and mobile push notification if you install the mobile app). Once you are assigned to the game, you will be required to accept or decline the assignment.
If the position that you requested is assigned to another referee, you will receive an email notification indicating that your request was not granted. Keep in mind that your assignor may have received multiple requests for the same game.