Why is a form field (or annotaion) created using PDFNet blank?

Q: I have an invisible control (an annotation) in my pdf document. How
do I make it visible using pdfnet api?
A: How do you create the annotation?

Because there is no standard way a form fields should look like, you
need to create its appearance (annot.SetAppearance()). For an example
of how to create a custom appearance you may want to take a look at
InteractiveForms sample project. You may also want to search this
forum using 'SetAppearance' for additional examples.

For some annotation types (such as text fields and simple types of
markup) you could simply call RefreshAppearance() which will
automatically generate a default appearance (and in this case it is
not necessary to call SetAppearance()).