How to assign (set) value of HTML TextArea from code behind in ASP.Net

1. Using runat server

Add runat="server"

<textarea id="commentBox" runat="server" cols="50" rows="8"></textarea>

And then in C#

commentBox.InnerText = "This is a test";

2. Without using runat server

Without runat="server" you will have too create a Protected Property above Page_Load event

protected string Value {get ; set;}
protected void Page_Load(object sender, EventArgs e)
{
   Value = "ABC";
} 

Then in HTML

<textarea id="commentBox" value="<%= Value%>" cols="50" rows="8"></textarea>