IdentityServer 4 with an old WebForms site - identityserver3

Question

I have an old webforms site which has many business critical users using microsoft.aspnet.identity 2.2
We are wanting to extend the site, but will be using a microservice type architecture to do this. We want to spin up new .NET core services, but they need to be able to connect to this legacy database full of users. And by this I mean be able to share logins (so the user doesn't need to login twice btn the various micro services)
Is the best way to standup a IdentityServer (on its own somewhere - maybe hosted in AWS) which connects to this database and then allow:
1) The webforms to authenticate to it
2) Future .net core micro services can auth to it?
Currently we have this for our app:
app.CreatePerOwinContext(ApplicationDbContext.Create);
app.CreatePerOwinContext<ApplicationUserManager>(ApplicationUserManager.Create);
app.CreatePerOwinContext<ApplicationSignInManager>(ApplicationSignInManager.Create);
// Enable the application to use a cookie to store information for the signed in user
// and to use a cookie to temporarily store information about a user logging in with a third party login provider
// Configure the sign in cookie
app.UseCookieAuthentication(new CookieAuthenticationOptions
{
AuthenticationType = DefaultAuthenticationTypes.ApplicationCookie,
LoginPath = new PathString("/Account/Login"),
SlidingExpiration = true,
CookieName = ".AspNet.SomeName"
});
// Use a cookie to temporarily store information about a user logging in with a third party login provider
app.UseExternalSignInCookie(DefaultAuthenticationTypes.ExternalCookie);h section

Answer 1

I would look at the IdentityServer4.Samples repo.
Looks like you are using OWIN, so you could easily adapt the MvcHybrid example:
https://github.com/IdentityServer/IdentityServer4.Samples/tree/release/Clients/src/MvcHybrid
If you are not using OWIN, then have a look at the HomeController in the MvcManual client. It can easily be ported over to a WebForms app.
https://github.com/IdentityServer/IdentityServer4.Samples/blob/release/Clients/src/MvcManual/Controllers/HomeController.cs