Autofill 2 Fields on a Web Page with C#

Question

I am working on a simple web forms application with C# (Microsoft Visual C# 2010 Express).

I have two text boxes (textBox1, richTextBox1) a button (button1) and a web browser (webBrowser1) on the form. The web browser goes to a web page when I run the program. On that page there are two input fields that I want to autofill with the click of the button1 using the text in textBox1 and richTextBox1.

You can see the code of the input fields on that web page:

<input type="text" id="subject" tabindex="4" name="subject" value="">

<textarea class="composebody" tabindex="6" name="message" id="message" rows="20" cols="80"></textarea>

I know this is very simple, but I don't have much knowledge about C#. Any ideas how I can code that?

Thanks.