The function LatestFile works in either Outlook VBA and Word VBA to give you the last file in any named folder.

You have said that you are creating a message from Word using VBA. The logical place to add the file is therefore to add the attachment using the macro that you use to create the message.
If you want to add it to the open message from Outlook, then the Word macro must first have completed, and as you seem reluctant to post that macro code, it is not possible to determine that is the case.

If that macro has finished, the following Outlook macro will add the last file from the named folder to the open message

Sub AddLatestFile()
'Graham Mayor - https://www.gmayor.com - Last updated - 28 May 2020 
Dim olMsg As MailItem
    On Error Resume Next
    Select Case Outlook.Application.ActiveWindow.Class
        Case olInspector
            Set olMsg = ActiveInspector.currentItem
        Case olExplorer
            Set olMsg = Application.ActiveExplorer.Selection.Item(1)
    End Select
    olMsg.Attachments.Add LatestFile("C:\path\folder\") 'Change the path as appropriate
lbl_Exit:
    Exit Sub
End Sub

Private Function LatestFile(strFolder As String, _
                    Optional strFilespec As String = "*.*") As String
Dim strName As String
Dim strRecent As String
Dim dDate As Date

    Do Until Right(strFolder, 1) = Chr(92)
        strFolder = strFolder & Chr(92)
    Loop
    strName = Dir(strFolder & strFilespec)
    If strName <> "" Then
        strRecent = strName
        dDate = FileDateTime(strFolder & strName)
        Do While strName <> ""
            If FileDateTime(strFolder & strName) > dDate Then
                strRecent = strName
                dDate = FileDateTime(strFolder & strName)
            End If
            strName = Dir
        Loop
    End If
    LatestFile = strFolder & strRecent
End Function