How to get Cloudfront subdomain from the request on the custom origin?

How does one get the AWS Cloudfront subdomain from the request on the custom origin that I've created.

The only header that comes close is the Via header but it doesn't contain any useful information

It looks like:

Via: 1.1 1026589cc7887e7a0dc7827b4example.cloudfront.net (CloudFront)

Note that 1026589cc7887e7a0dc7827b4example is not the original subdomain that I've assigned to the custom origin.