By default, HTML elements within an ASP.NET file are treated as literal text and you cannot reference them in server-side code. To make these elements programmatically accessible, you can indicate that an HTML element should be treated as a server control by adding the runat="server" attribute.
and Also
HTML server controls must reside within a containing form tag with the runat="server" attribute.
Reference: HTML Server Controls
Now you can access your control from code behind but here you can't get Text
property of input
html control, you need to get Value
property.
If you convert html page to ASPX then you can replace relevant ASP.NET control with existing HTML controls. for example you can use input type text with ASP.NET TextBox control