Try this
Private Sub HyperlinkMailtoWithDynamicSubject()
Dim objOL As Outlook.Application
Dim objNS As Outlook.Namespace
Dim objDoc As Word.Document
Dim objSel As Word.Selection
Dim currItem As mailitem
Dim MailtoSubject As String
Dim strHyperlinkAddress As String
Set objOL = Application
Set currItem = objOL.ActiveInspector.CurrentItem
MailtoSubject = currItem.subject
If objOL.ActiveInspector.EditorType = olEditorWord Then
Set objDoc = objOL.ActiveInspector.WordEditor
Set objNS = objOL.Session
Set objSel = objDoc.Windows(1).Selection
strHyperlinkAddress = "mailto:someone@somewhere.com?subject=" & MailtoSubject
objSel.Hyperlinks.Add Anchor:=objSel.Range, Address:=strHyperlinkAddress, _
TextToDisplay:="Please confirm here once email has been actioned"
End If
Set objOL = Nothing
Set objNS = Nothing
Set currItem = Nothing
Set objDoc = Nothing
Set objSel = Nothing
End Sub