var client = new MongoClient("mongodb://hostname/?connect=direct");

var adminDatabase = client.GetDatabase("admin");
var config = new BsonDocument
{
    { "_id", "rs" },
    { "members", new BsonArray
        {
            new BsonDocument
            {
                { "_id", 0 },
                { "host", "hostname:27017" }
            }
        }
    }
};
var command = new BsonDocument("replSetInitiate", config);
var result = adminDatabase.RunCommand<BsonDocument>(command);