To add a highlight annotation to a PDF Document page.

async function main() {
  const doc = await PDFNet.PDFDoc.createFromURL(filename);
  const page = await doc.getPage(1);

  // Create a highlight
  const hl = await PDFNet.HighlightAnnot.create(
    doc,
    new PDFNet.Rect(100, 490, 150, 515)
  );
  await hl.setColor(await PDFNet.ColorPt.init(0, 1, 0), 3);
  await hl.refreshAppearance();
  await page.annotPushBack(hl);
}
PDFNet.runWithCleanup(main);

Add or edit PDF annotations sample
Full code sample which shows how to add or edit PDF annotations (e.g. hyperlink, intra-document link, stamp, rubber stamp, file attachment, sound, text, free-text, line, circle, square, polygon, polyline, highlight, squiggly, caret, and ink).