Replace QR barcode with dynamically created QR barcode, ITextSharp

I have multi PDF forms that I have place QR barcode placeholders and would like to dynamically replace the QR barcode on those PDFS with a dynamically created QR barcode.
I have already done this
Dim qrcode As iTextSharp.text.pdf.qrcode.QRCode = New iTextSharp.text.pdf.qrcode.QRCode Dim codeText As String codeText = "FormID=" & formid & ":Form=" & filename & ":ID=" & applid & ":date=" & Now Dim qrcode2 As iTextSharp.text.pdf.BarcodeQRCode = New iTextSharp.text.pdf.BarcodeQRCode(codeText, 1, 1, Nothing)
Dim img As iTextSharp.text.Image img = qrcode2.GetImage()
img.SetAbsolutePosition(460, 750) cb.AddImage(img)
But, I don't want to hard code a X,Y coordinates.
I was working on something like this.
pdfStamper.AcroFields.ReplacePushbuttonField("barcodeNameonPDF",img) but I get errors.
Thanks,
mjc