I'm not sure how to do it the right way; here's a work-around:
Pass an environment variable to JMP
The echo command is making a tiny JSL file named test.jsl. The next line sets an environment variable named myvar and uses a & character to put a second command on the same line to start JMP with the test.jsl file. The jsl program uses the Get Environment Variable function to retrieve the value that set put in the environment variable.
Craige