asp:TextBoxes for passwords protect you from yourself!
I.e. the Text property is not rendered.
<asp:TextBox runat="server" ID="TextBox1" Text="Sample text" TextMode="Password" />
How to hack work-around it? Use the HTML value property.
<asp:TextBox runat="server" ID="TextBox1" value="Sample text" TextMode="Password" />
It’s lovely how ASP.NET looks after you.