// using default connection: mongodb://localhost/identityserver
identityServerBuilder
    .AddConfigurationStore()
    .AddOperationalStore();

const string connectionString = "mongodb://db.local.com/mydb";
identityServerBuilder
    .AddConfigurationStore(options =>
    {
        options.CollectionNamePrefix = "ids_";
        options.ConnectionString = connectionString;
    })
    .AddOperationalStore(options =>
    {
        options.CollectionNamePrefix = "ids_";
        options.ConnectionString = connectionString;
    });