Here is C# code that demonstrates how to create an empty interactive field for further signing of PDF document using digital signature: 
using System.Drawing;
using Vintasoft.Imaging.Pdf;

/// <summary>
/// Adds an empty signature field to a PDF page.
/// </summary>
public class PdfPageDigitalSignatureHelper_EmptySignatureField
{
    public static void Run()
    {
        // create a helper that alows to add digital signature to a PDF document
        PdfPageDigitalSignatureHelper digitalSignatureHelper =
            new PdfPageDigitalSignatureHelper(null, new RectangleF(20, 20, 250, 100));

        // set the appearance for signature field
        digitalSignatureHelper.SignatureText = "Add your signature here";
        digitalSignatureHelper.SignatureBackgoundColor = Color.Yellow;

        // open PDF document
        using (PdfDocument document = new PdfDocument("TestDocumentToSign.pdf"))
        {
            // add signature field to the first PDF page
            digitalSignatureHelper.AddEmptySignatureField(document.Pages[0]);

            // save PDF document to the output file
            document.SaveChanges("TestDocumentToSign-output.pdf");
        }
    }
}