Options for Ticket Portal Authentication for onprem

Has anyone successfully managed to setup access for customers to the ticket portal without using JWT authentication? 

We’re not a SaaS business, and our customers run the software on premises, often behind firewalls without access to the internet, so integrating intercom into the platform is a non starter. 

I’m looking at launching with Email & Slack channels, but have hit a blocker with the ticket portal feature - ideally I do not want to have to build a customer interface to surface this to customers.