How to protect ASP.NET sites behind a Windows password and forms authentication

You will also need to configure IIS and have your application point to IIS instead of the local web server

To configure Windows authentication

1. Start Internet Information Services (IIS).
2. Right-click your application's virtual directory, and then click Properties.
3. Click the Directory Security tab.
4. Under Anonymous access and authentication control, click Edit.
5. Make sure the Anonymous access check box is not selected and that Integrated Windows > authentication is the only selected check box.

http://msdn.microsoft.com/en-us/library/ms998358.aspx

To configure your application to point to IIS

1. Right click on your web project, and then click Properties
2. Click the Web tab
3. Click the "Use Local IIS Web Server" radio button

Assuming both sites share the same membership database then you can set the cookie domain in the forms authentication section of web.config;

<authentication mode="Forms">
    <forms .... domain="mycorp.com"/>
</authentication>

Note that you'll also have to setup matching machine keys in the web.config as these are used to sign the authentication cookie.