How do I obtain the IConfiguration parameter to instantiate a EF DbContext?

Question

I need to instantiate the following DbContext class:

public NWBContext(DbContextOptions<NWBContext> options, IConfiguration config) : base(options) {
    Config = config;
}

My current code to instantiate an instance of NWBContext is below.

string connectionString = connString;

var optionsBuilder = new DbContextOptionsBuilder<NWBContext>();
optionsBuilder.UseSqlServer(connectionString);

var config = new ConfigurationBuilder()                
.AddJsonFile(Path.Combine(AppDomain.CurrentDomain.BaseDirectory,"appsettings.json"))
.Build();

using (var dBContext = new NWBContext(optionsBuilder.Options, (IConfiguration)config)) {

}

This is for a class to import data from a CSV file into SQL. The import is too granular for a bulk insert and involves calculations that would be difficult in T-SQL.

The documentation at https://docs.microsoft.com/en-us/ef/core/dbcontext-configuration/#simple-dbcontext-initialization-with-new shows the following example:

var contextOptions = new DbContextOptionsBuilder<ApplicationDbContext>()
    .UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=Test")
    .Options;

using var context = new ApplicationDbContext(contextOptions);

The example does not address an IConfiguration parameter.

How would I instantiate the IConfiguration parameter to pass to the NWBContext constructor?

Some documentation implies that IConfiguration could be obtained from services. How would I obtain services?