How to turn HTML file into a Web Form
1. Rename file from .htm to .aspx in Solution Explorer
2. On top pf the page add <%@ Page Language="VB" %>
3. Close and reopen the file.
If you then want to add code behind file, look here: https://blogs.msdn.com/mikhailarkhipov/archive/2006/04/13/576117.aspx