Thank you all. I have added Show-Stream method in the activity and used the Rule-HTML-Harness with in that harness i have included the section. I tried with the Rule-HTML-Section directly and referred the section but it was showing without CSS only text no format available so have added harness.
#1. Added Not Auto Generated Section added script Windows.SetTimeOut(ShowAutoModal(),2)..wtihin settimeout function is having another function call ShowAutoModal()
#2. then set a variable
Var oSafeURL= new SafeURL("ClassName.ActivityName");
This activity is have the show stream line.
#3. Call pega library to show modal
Var showModalAuto = pega.OpenUrlInModal.showModalDialog(oSafeURL, '10000', '200','500');