Thanks for your answer and comments pointing me in the right direction.

Finally, I could find a solution by using this code:

renderer.getWriter().getAcroForm().addSignature("Signature1",50, 100, 400, 200 );

instead of:

var field = PdfFormField.createSignature(renderer.getWriter());
field.setWidget(new Rectangle(50, 100, 400, 200), PdfAnnotation.HIGHLIGHT_OUTLINE);
field.setFieldName("Signature1");
field.setFieldFlags(PdfAnnotation.FLAGS_PRINT);
renderer.getWriter().addAnnotation(field);
renderer.getWriter().addToBody(field);

Now the signature panel is filled correctly.