ASP.NET Core UseSetting from integration test

I have an integration tests project that uses .UseSetting() in the test class, as follows:

public AccessTokenRetrieval() : base(nameof(AccessTokenRetrieval))
{
    var connectionString = GetConnectionString();
    var dbSettings = new DbSettings(connectionString);
    _userGroupRepository = new UserGroupRepository(dbSettings);
    _roleRepository = new RoleRepository(dbSettings);
    _userRepository = new UserRepository(dbSettings);

    _server = new TestServer(new WebHostBuilder()
        .UseStartup<Startup>()
        .UseEnvironment("IntegrationTest")
        .UseSetting("IntegrationTestConnString", dbSettings.IdentityServerConnectionString));
    _handler = _server.CreateHandler();
    _client = _server.CreateClient();
}

I would now like to retrieve that setting in the Startup.cs of my actual project. I attempted to do so using:

public void ConfigureIntegrationTestServices(IServiceCollection services)
{
    var connectionString = Configuration.GetValue<string>("IntegrationTestConnString");

    BuildIdentityServerTests(services, connectionString);
    AddCoreServices(services, connectionString);
}

but that seems to return null.

What is the proper way to retrieve this setting?