Thanks for your response 

I made another scenario but with the same result

Retrieve field value then add username finally assign field value

the 2 ways make sense with single value 

but with multi-value in (Assign field values) i have to specify exactly the index of the field i want to store the value in and in my scenario it is variable it is the last index.

You will have to build your logic to determine which (if any) multivalue field has changed.  Once you have the changed multivalue field in a multivalue token, you can use the following script to add the initiator to the start of the data.

        Protected Overrides Sub Execute()
            'Write your code here.
            Dim sError As String = Nothing
            Dim sModified As String = "False"
            Dim oField as Object = GetTokenValue("MultiValueToken")
            Dim initiator as String = GetTokenValue("Initiator")
            Dim lstField As List(Of String) = New List(Of String)
            if oField.GetType.ToString = "System.String[]" Then
                Try
                    Dim lstVals As List(Of String) = New List(Of String)
                    For Each sVal As String In oField
                        lstVals.Add(sVal)
                    Next
                    Dim sLastVal As String = lstVals.Item(lstVals.Count - 1)
                    If Not sLastVal.StartsWith(initiator) Then
                        Dim newLastVal As String = initiator & " - " & sLastVal
                        oField.SetValue(newLastVal, oField.Length - 1)
                        sModified = "True"
                    End If
                Catch ex As Exception
                    sModified = "False"
                    oField = Nothing
                    sError = ex.Message
                End Try
            Else
                sModified = "False"
                oField = Nothing
                sError = "Field is not Multivalue"
            End If
            SetTokenValue("Modified", sModified)
            SetTokenValue("Error", sError)
            SetTokenValue("MultiField", oField)
        End Sub

Then if the script token Modified = true, replace template field value with script MultiField token.