Itextsharp setfield not replacing the text as expected in mail attachment .NET C# core

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I have a pdf template with below content

First Name: {{FirstName}}
Contact Number: {{ContactNumber}}
Email Id: {{EmailId}}
Address 1: {{Address1}}
Address 2: {{Address2}}
Qualification: {{Qualification}}


Using itextsharp i need to replace the texts which is inside the curly braces to its actual texts. 

But when i check the generated pdf, the texts in the {{}} are NOT getting replaced. Any idea why?

Update: i heard to know that AcroFields not a simple text, its a form field. Can anyone tell me how to create this acrofields in my template?

So for that i have coded like this

                    string formFile = path;

                    PdfReader reader = new PdfReader(formFile);
                    MemoryStream stream = new MemoryStream();
                    PdfStamper stamper = new PdfStamper(reader, stream);
                    AcroFields fields = stamper.AcroFields;

                    fields.SetField("{{FirstName}}", objUser.FirstName);
                    fields.SetField("{{ContactNumber}}", objUser.EmailId);
                    fields.SetField("{{Address1}}", objUser.AddressLine1);
                    fields.SetField("{{Address2}}", objUser.AddressLine2);
                    fields.SetField("{{Qualification}}", objUser.Education);
                    fields.SetField("{{AppNumber}}", "01A2021"+objUser.UserId.ToString());

                    stamper.Writer.CloseStream = false;

                    stamper.FormFlattening = true;
                    stamper.Close();

                    stream.Position = 0;
and then created my email attachment using

                    System.Net.Mime.ContentType contentType = new 
                    System.Net.Mime.ContentType(System.Net.Mime.MediaTypeNames.Application.Pdf);
                    var coverAttachment = new Attachment(stream, contentType);
                    coverAttachment.ContentDisposition.FileName = "CoverLetter.pdf";
                    emailMessage.Attachments.Add(coverAttachment);