You've posted your question to the FM Go forum which is intended to assist users who have FM GO installed on their iPads, iPhones and iPod touch devices. This appears to be a question about using a dos command with FileMaker Pro and that would have nothing to do with FileMaker GO correct?
A post of such a question to the FM Pro Forum (see tab above) would likely get read and responded by more people who can help you if you post there.
Why do you want to access this variable?
I think this is what they call an "enviromentment variable" If so and if this is a value set from outside FileMaker, I am unsure that it can be accessed from within FileMaker, but maybe you could set up a batch file that captures this value and exports it to a text file. Then FileMaker can use Send Event to run the batch file followed by using import records to import the text of the text file into a table in order to access the value. A major Kludge, but might work for you.