ASP.NET dynamically created textbox doesn't change value

I have the following issue: I create a TextBox dynamically in my web page, its value is "initialVal" in the beginning. Now I need to make a postback (not callback) to the server, and during this operation, I need to compute/change the value of my textbox to other value.

Here's an example:

protected void Page_Load(object sender, EventArgs e)
    {
        TextBox txtBox = new TextBox();
        txtBox.ID = "newButton";
        form1.Controls.Add(txtBox);
        txtBox.Text = "initialVal";

        if (IsPostBack && Session["change"] == null)
        {
            txtBox.Text = "change";
            Session["change"] = true;
        }
    }

The problem: even if I change the value via code, the textbox will keep the text "initialVal". I feel this is something related to the view state, but I don't understand. Coudl anyone please help me here?

Thanks.