In this iText 7 example, we'll create a text field and we'll add it to a PDF:
public void manipulatePdf(String src, String dest) throws IOException {
PdfReader reader = new PdfReader(src);
PdfDocument pdf = new PdfDocument(reader, new PdfWriter(dest));
PdfAcroForm form = PdfAcroForm.getAcroForm(pdf, true);
PdfFormField tf = PdfTextFormField.createText(
pdf, new Rectangle(110, 780, 70, 26), "text", "Text Field")
.setBorderColor(Color.BLUE)
.setBorderWidth(2)
.setColor(Color.RED)
.setFontSize(12);
form.addField(tf);
pdf.close();
}
Source: developers.itextpdf.com