Log into your Control Panel with your Hostpoint ID. Select the menu item "Admin" and then click "Hostpoint ID" and "Hosting access". Now select "Access right on my hostings" and select the hosting account you would like to authorize. Right below this item, select the Hostpoint ID of your web designer and click "Save".
Please note that an e-mail will be sent to your webmaster to notify him/her of activated authorization.