How do you add Identity server 4.0 as an external provider? - asp.net-core

Question

It looks like all of the sources talk about adding external providers INTO Identity Server 4, not using Identity Server 4 AS an external provider.
My startup.cs has this line for Facebook authentication:
services.AddAuthentication(options =>
{
options.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
options.DefaultChallengeScheme = OpenIdConnectDefaults.AuthenticationScheme;
})
.AddFacebook(facebookOptions =>
{
facebookOptions.AppId = "<appId>";
facebookOptions.AppSecret = "<appSecret>";
facebookOptions.SaveTokens = true;
})
I have a button on my login page:
<a class="btn btn-primary"
asp-action="ExternalLogin"
asp-route-provider="Facebook"
asp-route-returnUrl="">
Facebook
</a>
Which leads to:
[HttpPost]
[HttpGet]
[AllowAnonymous]
public IActionResult ExternalLogin(string provider, string returnUrl = null)
{
// Request a redirect to the external login provider.
var redirectUrl = Url.Action("ExternalLoginCallback", "Account", new { ReturnUrl = returnUrl });
var properties = _signInManager.ConfigureExternalAuthenticationProperties(provider, redirectUrl);
return Challenge(properties, provider);
}
Which all works.
I also have an Identity Server 4.0 server setup and I would like to use it as just another external provider.
services
.AddAuthentication(options =>
{
options.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
options.DefaultChallengeScheme = OpenIdConnectDefaults.AuthenticationScheme;
})
.AddFacebook(facebookOptions =>
{
facebookOptions.AppId = "<appId>";
facebookOptions.AppSecret = "<appSecret>";
facebookOptions.SaveTokens = true;
})
.AddIdentityServer(identityServerOptions=> //Doesn't exist? or Does it?
{
identityServerOptions.AppId = "<appId>";
identityServerOptions.AppSecret = "<appSecret>";
identityServerOptions.SaveTokens = true;
})
How do I add identity server as an external auth provider?
**UPDATE: **
From This site
#if (Model.ExternalProviders.Any())
{
<div class="row">
<div class="panel-body">
<ul class="list-inline">
#foreach (var provider in Model.ExternalProviders)
{
<li>
<a class="btn btn-default"
asp-action="ExternalLogin"
asp-route-provider="#provider.AuthenticationScheme"
asp-route-returnUrl="#Model.ReturnUrl">
#provider.DisplayName
</a>
</li>
}
</ul>
</div>
</div>
}
It looks like the provider is the same as the Authentication Scheme

Answer 1

Identityserver is a certified OpenId Connect provider, so you can use just:
.AddOpenIdConnect(authenticationScheme, displayName, options =>
{
options.SignInScheme = IdentityServerConstants.ExternalCookieAuthenticationScheme;
options.Authority = "<base-path-of-your-external-IS>";
options.ClientSecret = "<appSecret>";
options.ClientId = "<appId>";
options.ResponseType = OpenIdConnectResponseType.Code;
options.SaveTokens = true;
}