Error "Licensing Service Unavailable" When Activating Tableau Server Online via a Forward Proxy Server Without ATR Enabled

When activating a Tableau Server online without ATR enabled using a forward proxy server, the following error might occur:

Licensing Service Unavailable

Additionally, the following error might appear in the logs:

Tableau could not connect to the licensing server or FlexNet Operations Server. Verify the product key is correct and the primary or online licensing server can be accessed by ports 27000-27009. 

Cause

To access and subsequently configure product key information for your Tableau Server installation, not only the Tableau Server Administrative Controller but also Tableau Server Activation Service, must run under the Run As service account to go through the configured proxy server.