excel - Set default public variable at the first time and increase it in the next time?

I tried to write VBA script in outlook to increase row of cells in excel. But I cannot set default value for this variable (I use public variable and row of cell should be started from "2").

How can I use script VBA in outlook to set second row of cell ( cell(2,1) =1) in excel at the first time, and increase row of cell for the next time

Here is my code in outlook. and when I run it, It said that "Invalid outside procedure"

Public count As Integer: count = 2

Sub test()


    Dim xlApp As Object
    Dim xlWB As Object
    Dim xlSheet As Object

    On Error Resume Next
    Set xlApp = GetObject(, "Excel.Application")
    If Err <> 0 Then
        Set xlApp = CreateObject("Excel.Application")
    End If
    On Error GoTo 0

    'Check file open or not, and open it if it isn't opened
    If (IsWorkBookOpen("D:\Book1.xlsx") = True) Then
        Set xlWB = xlApp.Workbooks("Book1.xlsx")
    Else
        Set xlWB = xlApp.Workbooks.Open("D:\Book1.xlsx")
    End If

    Set xlSheet = xlWB.Sheets("Sheet1")

    'Process excel file

    xlSheet.Cells(count, 1) = 1
    count = count + 1

End Sub

Function IsWorkBookOpen(FileName As String) 'function to check excel file is open or not
    Dim ff As Long, ErrNo As Long
    On Error Resume Next
    ff = FreeFile()
    Open FileName For Input Lock Read As #ff

    Close ff
    ErrNo = Err
    On Error GoTo 0
    Select Case ErrNo
    Case 0:    IsWorkBookOpen = False
    Case 70:   IsWorkBookOpen = True
    Case Else: Error ErrNo
    End Select
End Function