Confusion with running code

I’ve done plenty of programming with C#, VB, HTML, and some others, but I’m pretty new to ASP.NET. I think I’m under a misconception and I cannot figure out where I am going wrong.

I downloaded Microsoft’s Visual Web Developer and started up a asp.net web application. I went into the file Default.aspx.cs file, added the reference System.Microsoft.Forms (so I can use Messagebox.Show() don’t ask… I just like it), I put in the code:


private void test123()
{
MessageBox.Show("test");
}

I then went into the html and put in:


<p><% test123(); %></p>

When this page runs or i click on the tex or something… shouldn’t a message box appear? I’m under the assumption that when you put the test123() in between <% %> it will go into the default.aspx.cs page and run the code for test123(). How would I actually get that code to run?

Thanks