How do you get the connection string from the Startup.cs

How do you get the connection string from the startup.cs

public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

// code -----------------------------

            // Add our db
            services.AddDbContext<NextGenerationDbContext>(options =>
                options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));

            // Add application services here
            services.AddTransient<IServiceAuth, ServiceAuthc>();

    }

In my appsettings.json ... "DefaultConnection: "Server=my string";

Here is my static connection class:

 public static class Connection
    {
        public static string ConnectionString { get; set; }

        public static SqlConnection ConnectionFactory()
        {
            return new SqlConnection(ConnectionString);
        }
    }

Here is the method I'm trying to pass the string too. This a service.

public int InsertPersonActivityLog(int logId, string edi, string personFormalName, DateTime activityDateTime, string activityTypeDescriptor, string additionalDetails, string reasonMessage, string notes)
        {
            SqlConnection conn = new SqlConnection();

            // Get the connection string from the appsettings.Development.json

            conn.ConnectionString =  // here is where I'm trying to pass my connectionstring.
            // Open the connection
            conn.Open();

            // Set connection property of command object.
            SqlCommand cmd = new SqlCommand();
            cmd.Connection = conn;

            // Set the command type of the command object.
            cmd.CommandType = System.Data.CommandType.StoredProcedure;

            // Assign values as "parameters to avoid 'SQL Injections.'"
            cmd.Parameters.AddWithValue("@edi", edi);
            cmd.Parameters.AddWithValue("@formalName", personFormalName);
            cmd.Parameters.AddWithValue("@date", activityDateTime);
            cmd.Parameters.AddWithValue("@descriptor", activityTypeDescriptor);
            cmd.Parameters.AddWithValue("@details", additionalDetails);
            cmd.Parameters.AddWithValue("@reason", reasonMessage);
            cmd.Parameters.AddWithValue("@notes", notes);

            return cmd.ExecuteNonQuery();
        }