Ok I had thought I had found it, but playing around with different objects. You can add a textbox directly to the sheet through the sheets drawing notes. See below -
Sub main
Dim oDoc As DrawingDocument = ThisApplication.ActiveDocument
Dim OTG As TransientGeometry = ThisApplication.TransientGeometry
Dim oSheet As Sheet = oDoc.Sheets(4)
Dim InputText As String = "Testing textboxes"
Dim TextPoint As Point2d = OTG.CreatePoint2d(5, 5)
Dim iNote As DrawingNote = oSheet.DrawingNotes.GeneralNotes.AddFitted(TextPoint, InputText)
End Sub
The GeneralNotes has two different options identical to how we would create textboxes in other areas. Add fitted and Add by rectangle.