[c#] How do I reference the input of an HTML <textarea> control in codebehind?

I'm using a textarea control to allow the user to input text and then place that text into the body of an e-mail. In the code behind, what is the syntax for referencing the users input? I thought I could just use message.Body = test123.Text; but this is not recognized.

HTML:

<textarea id="TextArea1" cols="20" rows="2" ></textarea>

CodeBehind:

foreach (string recipient in recipients)
{         
  var message = new System.Net.Mail.MailMessage("[email protected]", recipient);
  message.Subject = "Hello World!";         
  message.Body = test123.Text;                
  client.Send(message); 
} 

First make sure you have the runat="server" attribute in your textarea tag like this

<textarea id="TextArea1" cols="20" rows="2" runat="server"></textarea>

Then you can access the content via:

string body = TextArea1.value;

You need to use runat="server" like this:

<textarea id="TextArea1" cols="20" rows="2" runat="server"></textarea>

You can use the runat=server attribute with any standard HTML element, and later use it from codebehind.

Missed property runat="server" or in code use Request.Params["TextArea1"]

You should reference the textarea ID and include the runat="server" attribute to the textarea

message.Body = TextArea1.Text;  

What is test123?