ASP.NET FAQ : How can I provide a default value for a password TextBox?
When a TextBox control has its TextMode property set to Password, you cannot
assign a default value to the control using the standard Text property. In order
to assign a default value, you need to use code that looks like this:
C# Version
txtPassword.Attributes["value"] = "default value";
VB.NET Version
txtPassword.Attributes("value") = "default value"