How to add a div with content using asp.net

I have created a website with a working registration and login. However, I ran into some trouble when trying to make a little forum. What I want the forum to do is creating a div containing the message when the user filled a textbox and submitted it by clicking on a button.

The question is, how can I add html to my div by just using code? This is what I have already.

protected void Submit_Click(object sender, EventArgs e)
    {
        if (Message.Text != null)
        {
            HtmlGenericControl post = new HtmlGenericControl("div");
            HtmlGenericControl message = new HtmlGenericControl("p");
            message.InnerText = Message.Text;
            post.Controls.Add(message);
        }
        else
        {

        }
    }

Any help or suggestions are appreciated, thank you.