How to set ASPNETCORE_ENVIRONMENT variable in VSO deploy template?

It is just an environment variable, and the Variables in the TFS Scripted build/releases are set as environment variables. You can set the from either the Variables tab in Build / Release when editing a definition: <img src="https://i.stack.imgur.com/Lfy28.png" alt="enter image description here"> or per Environment in a Release: <img src="https://i.stack.imgur.com/QP45a.png" alt="enter image description here">