In order to ensure that your Application Key is not misused the JavaScript API requires you to authorize what web domains can use your Application Key.
Note these are the domains you own not the ones you are taking screenshots of. So if you owned a website called example.com, you would add this to your authorized domains.
To authorize your domains go to the Authorized Domains page and add all the domains that are allowed to use the GrabzIt JavaScript API. Once you have added the domains wait a few minutes for the changes to take effect.
If you don't want to have any domain security just go to Authorized Domains and un-tick the Authorized Domain Security checkbox.