To set an annotation style property such as color, interior color, border style, padding, etc to an annotation.

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

  const circle = await PDFNet.CircleAnnot.create(
    doc,
    new PDFNet.Rect(100, 100, 200, 200)
  );
  await circle.setColor(await PDFNet.ColorPt.init(0, 1, 0), 3);
  await circle.setInteriorColor(await PDFNet.ColorPt.init(0, 0, 1), 3);
  const dash = [2, 4];
  await circle.setBorderStyle(
    await PDFNet.AnnotBorderStyle.createWithDashPattern(
      PDFNet.AnnotBorderStyle.Style.e_dashed,
      3,
      0,
      0,
      dash
    )
  );
  await circle.setPadding(new PDFNet.Rect(2, 2, 2, 2));
  await circle.refreshAppearance();
  await page.annotPushBack(circle);
}
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).