(ES3) Adding Invisible Signature field?

I understand that Signature Service can't Add Invisible Signature Field to a Dynamic XFA PDF, so I'm trying the following:

1. Forms Service's renderPDFForm operation to Merge Form and Data; and then
2. Output Service's transformPDF operation to Flatten PDF.

However I still get the error: ALC-DSS-300-007 Addition of Signature Fields is not supported for Dynamic XFA PDFs

This seems to indicate that previous steps I took do not in fact transform a Dynamic XFA PDF to Static PDF.

Is there are way to transform a Dynamic XFA PDF to a Static PDF so that I can add an invisible signature field that can be used by the Signature Service's Certify PDF operation?

(I feel like I must be overlooking something simple, and my google-fu is failing me.)