Migrating from asp.net to asp.net core

I have an existing implementation of component space Saml 2 for asp.net (SAML v2.0 for .NET4.6) v2.8.0.0 which uses the ServiceProvider.SendAuthnRequestByHTTPRedirect(), which uses the AuthnRequest model. it populates that AuthnRequest based on custom configurations from the DB.

I am trying to replace this logic in aspnetcore project. My problem is that I don’t see a way to send an AuthnRequest. I see what the documentation has described the ability to Initiate SSO request and set the configuration

await _samlServiceProvider.SetConfigurationIDAsync(domain);
await _samlServiceProvider.InitiateSsoAsync();

But I don’t see a way to actually send an AuthnRequest.

The other question or concern I have is that I don’t see and an equivalent way to map the properties on AuthnRequest to a SamlConfiguration. The reason I would like to go this route is that our existing implementation builds and AuthnRequest, not a SamlConfiguration

ServiceProvider.SendAuthnRequestByHTTPRedirect is part of the SAML low-level API for the ASP.NET product.
Generally we recommend using the SAML high-level API as this is simpler to use and requires less code in your application.
If you’re moving to ASP.NET Core I strongly recommend that you use the equivalent to the high-level API.
In other words, await _samlServiceProvider.InitiateSsoAsync(); etc.
InitiateSsoAsync will construct and send the SAML authn request.
It’s contents are controlled though SAML configuration and the parameters you supply.
If there’s something specific you need to do that can’t be done through the existing API, please let us know.