Connect to Dynamics 365 / DataVerse API in 2023

static void Main(string[] args)
        {
            IOrganizationService oServiceProxy;

            try
            {
                string ConnectionStringSecret = "AuthType=ClientSecret; " +
                  "ClientId=<<APP ID FROM AZURE>>; " +
                  "ClientSecret=<<CLIENT SECRET>>; " +
                  "Url=https://<<YOUR ORG>>.crm<<REGION NUMBER>>.dynamics.com/;";

                ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

                //Create the Dynamics 365 Connection:
                CrmServiceClient crmServiceClient = new CrmServiceClient(ConnectionStringSecret);

                if (!crmServiceClient.IsReady)
                {
                    Console.WriteLine(crmServiceClient.LastCrmError);
                }

                //Create the IOrganizationService:
                oServiceProxy = (IOrganizationService)crmServiceClient.OrganizationWebProxyClient != null ? (IOrganizationService)crmServiceClient.OrganizationWebProxyClient : (IOrganizationService)crmServiceClient.OrganizationServiceProxy;            

                Console.WriteLine("Validating Connection");

                if (oServiceProxy != null)
                {
                    //Get the current user ID:
                    Guid userid = ((WhoAmIResponse)crmServiceClient.Execute(new WhoAmIRequest())).UserId;

                    if (userid != Guid.Empty)
                    {
                        Console.WriteLine("Connection Successful!");
                    }
                }
                else
                {
                    Console.WriteLine("Connection failed...");
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine("Error - " + ex.ToString());
            }

            Console.ReadKey();
        }

Gotchas –

1. line 10, the MS docs say the url format is https://<<your org>>.crm.dynamics.com – you actually need the region code too, e.g. https://yourEUROPEorg.crm4.dynamics.com or https://yourUKorg.crm11.dynamics.com
2. this line is sometimes and seemingly randomly required prior to the creation of the CrmServiceClient to ensure the correct protocol is used.