How to set the Default Page in ASP.NET?
Tip #84: Did you know… How to set a Start page for your Web Site in Visual Web Developer?
Simply right click on the page you want to be the start page and say "set as start page".
As noted in the comment below by Adam Tuliper - MSFT, this only works for debugging, not deployment.
If using IIS 7 or IIS 7.5 you can use
<system.webServer>
<defaultDocument>
<files>
<clear />
<add value="CreateThing.aspx" />
</files>
</defaultDocument>
</system.webServer>
https://docs.microsoft.com/en-us/iis/configuration/system.webServer/defaultDocument/