Use different SignUpSignIn Azure AD B2C policies, depending on the hostname, allowing you to provide SSO for specific companies, without having to provide a button for each SSO integration on the main login page

ServiceCollectionExtensions.cs

using Microsoft.AspNetCore.Authentication.OpenIdConnect;

namespace AuthTest.Extensions;

public static class ServiceCollectionExtensions
{
    // USAGE:
    //
    // 1. Add the following line in Program.cs, after your call to AddMicrosoftIdentityWebApp:
    // builder.Services.AddAlternateSignUpSignInPolicies(builder.Configuration.GetSection("AzureAdB2C"));
    //
    // 2. Add a AlternateSignUpSignInPolicyIds subsection your appsettings.json, mapping host names to policy IDs:
    // "AzureAdB2C": {
    //     ... OTHER FIELDS OMITTED ...
    //     "AlternateSignUpSignInPolicyIds": {
    //         "thirdparty.local": "B2C_1_signin_ExampleCo"
    //     }
    // },
    //
    
    public static void AddAlternateSignUpSignInPolicies(this IServiceCollection services, IConfigurationSection configurationSection)
    {
        var defaultPolicyId = configurationSection["SignUpSignInPolicyId"];

        var alternateSignUpSignInPolicyIds = configurationSection
            .GetSection("AlternateSignUpSignInPolicyIds")
            .GetChildren()
            .ToDictionary(config => config.Key.ToLower(), config => config.Value);

        services.Configure<OpenIdConnectOptions>(OpenIdConnectDefaults.AuthenticationScheme, options =>
        {
            options.Events = new OpenIdConnectEvents
            {
                OnRedirectToIdentityProvider = context =>
                {
                    var host = context.Request.Host.Host.ToLower();
                    var altPolicyId = alternateSignUpSignInPolicyIds.GetValueOrDefault(host);
                    if (altPolicyId != null)
                    {
                        context.ProtocolMessage.IssuerAddress = context.ProtocolMessage.IssuerAddress
                            .Replace(defaultPolicyId, altPolicyId, StringComparison.OrdinalIgnoreCase);
                    }
                    return Task.CompletedTask;
                }
            };
        });
    }
}