public class AppOptions
{
    public string RedisConnectionString { get; set; } = "localhost:6379";
}

services.Configure<AppOptions>(Configuration);
services.AddSingleton<IConnectionMultiplexer,ConnectionMultiplexer>(sp =>
{
    var settings = sp.GetRequiredService<IOptions<AppOptions>>().Value;
    var configuration = ConfigurationOptions.Parse(settings.RedisConnectionString, true);

    return ConnectionMultiplexer.Connect(configuration);
});

private readonly IDatabase _database;
public AccountController(IConnectionMultiplexer connectionMultiplexer)
{
    _database = connectionMultiplexer.GetDatabase();
}

public IActionResult Index()
{
    _database.StringSet("key", "value");
    return View();
}