Is there a document describing how the RDS external connector can be activated under Windows server 2019?
There is a document describing the activation under server 2008: https://support.microsoft.com/en-us/help/887432/how-to-install-and-to-configure-the-external-connector-for-a-terminal
The same settings under 2019 gives in de RDS session a popup with the text: There is a problem with your Remote Desktop license, and your session will be disconnected in 60 minutes.