This sample shows how to add text fields to your PDF document using PdfPage.AddTextBox(..) method.

AddTextBox method returns an instance of PdfTextBox class.

C# 
using System.Diagnostics;

namespace BitMiracle.Docotic.Pdf.Samples
{
    public static class TextFields
    {
        public static void Main()
        {
            // NOTE: 
            // When used in trial mode, the library imposes some restrictions.
            // Please visit http://bitmiracle.com/pdf-library/trial-restrictions.aspx
            // for more information.

            string pathToFile = "TextFields.pdf";

            using (PdfDocument pdf = new PdfDocument())
            {
                PdfPage page = pdf.Pages[0];
                PdfTextBox textBox = page.AddTextBox(10, 50, 100, 30);
                textBox.Text = "Hello";

                PdfTextBox textBoxWithMaxLength = page.AddTextBox(10, 90, 100, 30);
                textBoxWithMaxLength.MaxLength = 5;

                PdfTextBox passwordTextBox = page.AddTextBox(10, 130, 100, 30);
                passwordTextBox.Text = "Secret password";
                passwordTextBox.PasswordField = true;

                pdf.Save(pathToFile);
            }

            Process.Start(pathToFile);
        }
    }
}
Visual Basic 
Imports System.Diagnostics

Imports BitMiracle.Docotic.Pdf

Namespace BitMiracle.Docotic.Pdf.Samples
    Public NotInheritable Class TextFields
        Public Shared Sub Main()
            ' NOTE: 
            ' When used in trial mode, the library imposes some restrictions.
            ' Please visit http://bitmiracle.com/pdf-library/trial-restrictions.aspx
            ' for more information.

            Dim pathToFile As String = "TextFields.pdf"

            Using pdf As New PdfDocument()

                Dim page As PdfPage = pdf.Pages(0)
                Dim textBox As PdfTextBox = page.AddTextBox(10, 50, 100, 30)
                textBox.Text = "Hello"

                Dim textBoxWithMaxLength As PdfTextBox = page.AddTextBox(10, 90, 100, 30)
                textBoxWithMaxLength.MaxLength = 5

                Dim passwordTextBox As PdfTextBox = page.AddTextBox(10, 130, 100, 30)
                passwordTextBox.Text = "Secret password"
                passwordTextBox.PasswordField = True

                pdf.Save(pathToFile)
            End Using

            Process.Start(pathToFile)
        End Sub
    End Class
End Namespace