Restricting ticket submission to certain domains by Brand

We use Zendesk both for communicating with the public and for internal tech support ticket submissions. I want to set up a publicly accessible Help Centre using Zendesk Guide for Internal tech support, and while I don't necessarily care who logs in to it - I don't want just anyone with a Hotmail account submitting tickets. What I'd like to be able to do is restrict ticket submission by Brand, so that the tech support Brand can only receive tickets from X while the customer service Brand can receive tickets from anyone.