Add acroform using pdfsharp

How can I add Acroforms (or any input fields) with pdfsharp lib to pdf? For example a textfield (PdfSharp :: Pdf :: AcroForms :: PdfTextField) I can't find any example for this, read / modify only.

I found "page-> Elements-> Add (key, pdfitem)" but I cannot create an object from PdfSharp :: Pdf :: AcroForms :: PdfTextField or other forms (no constructor)