Request property default value

Environment: 1IM 9.1.1 Angular WebPortal

we are adding 2 custom Request Properties for a service item 

StartDate and ExitDate

both of'm are DateTime type and UserPrompt, i would like to have the value of ExitDate automatically set to StartDate + 1 year whenever the StartDate is modified in WebPortal (it also can be manually modified later)

For this im using the "Script for Changing Values" in StartDate with this code

Dim startDate? = DbVal.ConvertTo(Of DateTime)(Value.ToString())
If startDate.HasValue And startDate.GetValueOrDefault() <> DbVal.MinDate Then

      ParameterSet("ExitDate").Value= startDate.GetValueOrDefault().AddYears(1).AddDays(-1)

End If

but the value for ExitDate is never updated in the corresponding WebPortal control 

Is the sintax ParameterSet("ExitDate").Value correct ? i got it from It Shop Administration manual