Here is a code sample:

using System;
using System.Collections.Generic;
using Skyline.DataMiner.Automation;
using Skyline.DataMiner.Library.Reservation;
using Skyline.DataMiner.Library.Solutions.SRM;

public class Script
{
public static void Run(Engine engine)
{
// Replace with reservation guid
var reservationId = Guid.NewGuid();

var properties = new Dictionary<string, object> { { "testproperty", "testvalue" } };

var reservationInstance = SrmManagers.ResourceManager.GetReservationInstance(reservationId);
reservationInstance.UpdateServiceReservationProperties(properties);
}
}