z/OS customers use the DYLPARM keyword discussed on pages 50/51 of the VISION:Results for z/OS reference Guide r6.
VSE customers can use the DYLSYSPARM keyword to pass a parameter from the JCL as documented on page 23 of the VISION:Results Reference Guide Release 5.0
For example, this would pass the value "PR" to the field WK_PARM:
// OPTION SYSPARM='PR'
WK_PARM 8 CH
MOVE DYLSYSPARM TO WK_PARM
VSE customers must also set the DYLINSTL customizing parameter EXTEND to Y. This sets up both extended fields DYLSYSDATE and DYLSYSPARM from the communications region. The default is N, so these fields are otherwise blank.