uint repeatCount = ...;
    using (var netscapeData = new MemoryStream(4))
    using (var writer = new BinaryWriter(netscapeData))
    {
        writer.Write((byte)3); // Number of following data bytes
        writer.Write((byte)1); // Always 1
        writer.Write(repeatCount);
        writer.Flush();

        var containerProperties = new BitmapPropertySet
        {
            { "/appext/Application", new BitmapTypedValue("NETSCAPE2.0", PropertyType.String) },
            { "/appext/Data", new BitmapTypedValue(netscapeData.ToArray(), PropertyType.UInt8Array) },
        };
        await encoder.BitmapContainerProperties.SetPropertiesAsync(containerProperties);
    }