A trouble with VS Code

[Truman]

I'm on this page now:
https://flask.palletsprojects.com/en/1.1...l/factory/

When I use Notepad++ and console have no problem in activating the environment and web page http://127.0.0.1:5000/hello. But...when I try the same in VisualStudio Code I get this message:

Error:
Error: Could not locate a Flask application. You did not provide the "FLASK_APP" environment variable, and a "wsgi.py" or "app.py" module was not found in the current directory.

any idea what's behind this? How do you enter env in VS Code?