I searched around and found about GetInspector object. That page has an example in VBA that I have translated to AHK and as suggested by ruespe I'm sharing my solution here as well.
; Example of creating a message keeping the default signature olMailItem := 0 MailItem := ComObjActive("Outlook.Application").CreateItem(olMailItem) myInspector := MailItem.GetInspector wdDoc := myInspector.WordEditor wdRange := wdDoc.Range(0, wdDoc.Characters.Count) wdRange.InsertBefore("This is the body.") MailItem.Display