Add Environment Variable to MSIX package using PSF Fixups in Packaging Tool - what am I missing?

Your syntax looks OK to me. 

I will mention that you will not see the environment variable by using any tools running outside of the container.  Also, with userregistry = false you would also not see it in a process run inside the container if that process was not launched by PsfLauncher, which would be any process other than randomapp.