How to access REST via a C# control, self-hosted

Consider this a question that I should probably know the answer to. Client is self-hosted on 20.2 SPV, won't be 20.3 or 2020 for a while. Building a simple page to update their personal profile (yes, would prefer to use a simple panel but you know clients) using a simple C# control. I know how to access REST from an Azure function, and how to access it via JavaScript, but I'm having a hard time accessing it using C# code-behind. I can't access the JavaScript __RequestVerificationToken value, and I really don't want to embed a full-user account (don't need to do anything but let someone update their own account). What's the trick? iMIS uses REST (right?) and there isn't an account embedded anywhere that's being used (that I know of). Any advice would be helpful, thanks.