Add Text Field Annotation in a document | https://docs.conholdate.com/net/add-text-field-annotation/

add-text-field-annotation.cs

//Add text field annotation to the document from local disk
using (Annotator annotator = new Annotator("input.pdf"))
{
	TextFieldAnnotation textField = new TextFieldAnnotation
    {
    	BackgroundColor = 65535,
        Box = new Rectangle(100, 100, 100, 100),
        CreatedOn = DateTime.Now,
        Text = "Some text",
        FontColor = 65535,
        FontSize = 12,
        Message = "This is text field annotation",
        Opacity = 0.7,
        PageNumber = 0,
        PenStyle = PenStyle.Dot,
        PenWidth = 3,
        FontFamily = "Arial",
        TextHorizontalAlignment = HorizontalAlignment.Center,
        Replies = new List<Reply>
        {
        	new Reply
            {
            	Comment = "First comment",
                RepliedOn = DateTime.Now
            },
            new Reply
            {
            	Comment = "Second comment",
                RepliedOn = DateTime.Now
            }
        }
	};
    annotator.Add(textField);
    annotator.Save("result.pdf");
} 

Read the complete article: https://docs.conholdate.com/net/add-text-field-annotation/