In Windows, an environment variable is set through graphical interface and the variable refers to another one. The varible that is refered cannot have a refrence to yet another. It seems that in the command line it is only expanded one level. In the graphical interface the variables may look ok, but they are not going to be fully expanded. In the registry they all have the value: REG_EXPAND_SZ. The refered varible MUST be only REG_SZ. Maybe it works in Windows environment but for use in the command line it doesn’t.
plink -L 127.0.0.1:13389:scl_machine:3389 firstname.lastname@example.org