Mixed authentication is relatively easy to do. I do suggest searching for examples. I'll briefly explain what I did about 5 years ago.
1. Set your web.config to use Forms Authentication.
https://msdn.microsoft.com/en-us/library/7t6b43z4(v=vs.140).aspx[
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2. Set the login url to a page that will have code which attempts to get the user id from HttpContext.Current.Request.ServerVariables["LOGON_USER"] which is how it will be if windows authentication is turned on.
3. Then enable windows authentication on that one single page. And in the code of that windows logon page if it fails, redirect to a regular LoginPage which will ask for the username and password.
So, essentially what happens is when a request comes in, ASP.Net looks at web.config and sends the user to WindowsLogin.aspx (or whatever you name it). In that page, attempt to get user id from browser and if validated, login them in with FormsAuthentication. If it failes, redirect to a regular LoginPage where they can type in name and password.