Pass public variable to Sub

[gmayor]

As Paul says, you need to create a new module as he has shown and you put the line (and nothing else required) in that new module.
The macro

Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
    strSubject = Item.Subject
    MsgBox (strSubject)
    UserForm1.Show
End Sub

without that line goes in the ThisOutlookSession module, shown at the top of the folder list in Paul's illustration.
The macro

Private Sub ButtonOK_Click()
    MsgBox strSubject
End Sub

goes in the Userform code. The userform will then show when you click 'Send' on a message.