Hi @db042190
OK, I see what you mean. I simply did a test.
First I created a hidden parameter "call" with the available values set to "M" and "S" as you meant.
Next a date parameter "date" is created whose default value depends on the parameter "call". I used the IIF function, when the value of "call" is "S", the date of "date" is set to "today-30 days", otherwise it is empty.
=iif(Parameters!call.Value="S",DateAdd(DateInterval.day, -30, today()),"")
After deploying the report, create a standard subscription. Then we can see that when we select the hidden parameter "call" as "S", the value of "date" is automatically set to "today-30 days".
Hope this can help you.
Best regards,
Aniya