To add a field to a PDF interactive form is necessary to do the following steps:

• Create an object defining the field data
• Define the form appearance using standard parameters OR using graphical commands
• Add the object defining the field data to a list of form fields
• Obtain a widget annotation defining the field appearance
• Add the widget annotation to a list of annotations on the page where the field have to be located

Please see concrete examples here:

• Add a check box field to the PDF interactive form
• Add a pushbutton field to the PDF interactive form
• Add a group of radio buttons field to the PDF interactive form
• Add a combo box field to the PDF interactive form
• Add a list box field to the PDF interactive form
• Add a text input field to the PDF interactive form
• Add a barcode field to the PDF interactive form
• Add a signature field to the PDF interactive form