how to develop a widget for my website?

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C#

ASP.NET

hi everyone,
i am using a asp.net, In my website i am provide a code for a user advertisement, the user can copy & paste a coding in his website which i am provide.
here i am usng a <iframe> to generate a code.</iframe>.

protected void Button2_Click(object sender, EventArgs e)
    {
        Preview.Visible = true;   //i am using panel to preview his design. 
        //string bl = txt_fore.Text;
        //string bg = txt_back.Text;
        string htmlcode ="";

        htmlcode += "<iframe allowtransparency=" + "true" + " " + "frameborder=" + "1" + " " + "height=" + "250px" + " " + " width=" + "140px" + " " + "hspace=" + "0" + " " + "scrolling=" + "no" + " " + " src=sam.html></iframe>";

  txt_modtext.Text = htmlcode;// here i am generate a code.and passed in textbox.

i am collect three info from user:

1. label1.text= Offername;
2. label2.text= description about offer;
3. label3.text= valid date;

above 3 are collect in my website, how i put into between iframe.
but i got a problem in that, how can i place user Details in that i am frame.