I created a new internet facing NLB with a VPC endpoint service with allowedprinicipals = internal in our Beta account. Need help to test it.

I am assuming you are using a service endpoint hosting as a service provider as mentioned in below document and allowing "INTERNAL" users to access the service.

https://docs.aws.amazon.com/vpc/latest/privatelink/privatelink-share-your-services.html

There are service consumer i.e interface endpoint to use the service. All following steps are mentioned in above document.