(ASP.NET) Changing an input at the start of Page_Load() and then retriving the input value after form submit gives the old value

I have a form that is supposed to have defalt values, which the user can decide to change in some fields. The inputs are set up like this:

<input type="text" id="username" clientidmode="static" runat="server" />

I am setting the default data at the start of Page_Load() like this:

username.Value = "test"

And I’m getting the values later in if(isPostBack) like this:

string sUsername = username.Value;

The idea is that they will be able to change certain values about the user, or just leave the default values if they don’t want to change that field.

The issue is that when I’m getting the field values in if(isPostBack) I’m just getting the values I’ve set at the start of Page_Load() even if the user changed the form values.

What am I doing wrong?

Here is a simple way to reproduce this. Create an asp.net project and add a WebForm.

Then add this to the page:

<input type="text" id="username" runat="server" />
<input type="submit" />

and add this to the code behind:

protected void Page_Load(object sender, EventArgs e)
{
     username.Value = "Test";

     if (IsPostBack)
     {
          string sUsername = username.Value;
     }
 }