There is no way to send a parameter. Two workaround I can think of would be:
Create a property on the case and then set that before running the correspondence that references the property. If you do this be sure to check the "Do not save property data" option on the advanced tab of the property so you don't bothe persisting it to the DB.
The other way would be to use the Prompt tab of the correspondence but that prompts the end user which may not be at all what you want.
Cecil Howell, CSA | Senior Instructor | Pegasystems Inc.
If you are using any page directly under work class to send correspondence, to handle temporary data create a property and check "Do not save property data" check box in advanced tab. Set parameter to this property and use it correspondence, the property will not be stored into blob.