How to open the upper left context menu via keystroke (for PuTTY on Windows)

You need to set System menu appears on ALT-Space or System menu appears on ALT alone in PuTTY's Windows | Behaviour settings.

Just adding to the answer from Mike Fitzpatrick

In order for the changes to be effective, you need to:
Open the context menu (upper left) > Click Change Settings... > Windows (Left Pane) > Behaviour
Tick one of the items "System menu appears on.."
and click "Apply"