Hi Ruben,

It is highly advised not to use the SRM request messages directly, but rather use the helpers, such as ResourceManagerHelper in SLNet. It does have the API calls to create or update a Resource.
Pseudo code example:

public void Run(Engine engine)
{
    var rmHelper = new ResourceManagerHelper(engine.SendSLNetSingleResponseMessage);
    var protocolFunctionHelper = new ProtocolFunctionHelper(engine.SendSLNetSingleResponseMessage);
    var profileHelper = new ProfileHelper(engine.SendSLNetSingleResponseMessage);

    var mainElement = engine.FindElement("IRD Main Decoder");
    var functionElement = engine.FindElement("IRD Satellite Decoder");
    FunctionDefinitionID functionID = new FunctionDefinitionID(Guid.Parse("9fca935f-2794-4d9c-aa30-016b73ab4544")); // function definition ID example
    SystemFunctionDefinition functionDefinition = protocolFunctionHelper.GetFunctionDefinition(functionID, searchSystemFunctionDefinition: false); // function definition example
    ResourcePool decoderPool = rmHelper.GetResourcePool(Guid.Parse("eb08eb7f-93bc-4b80-943a-f8055c772e10")); //Decoder pool example
    Parameter capacityParameter = profileHelper.ProfileParameters.Read(ParameterExposers.ID.Equal(Guid.Parse("4fa4d673-c5fd-4477-8551-078d4be09f9f"))).FirstOrDefault(); // Capacity parameter example
    Parameter capabilityParameter = profileHelper.ProfileParameters.Read(ParameterExposers.ID.Equal(Guid.Parse("30067237-ba12-4ad3-b2c4-2bda53d406c4"))).FirstOrDefault(); // Capability parameter example

    var tableParameter = 2000;  //linked table parameter
    List<FunctionEntryPoint> entryPoints = protocolFunctionHelper.GetFunctionEntryPoints(functionID.Id, functionElement.DmaId, functionElement.ElementId);
    FunctionEntryPoint entryData = entryPoints.FirstOrDefault(x => x.ParameterID == tableParameter && x.Data?.Length > 0);
    EntryPointData nextEntryData = entryData.Data.FirstOrDefault(x => x.Element == null); // next unused table entry

    var decodeFunctionResource = new Skyline.DataMiner.Net.ResourceManager.Objects.FunctionResource(mainElement.DmaId, mainElement.ElementId)
    {
        GUID = Guid.NewGuid(),
        Name = "IRD Decoder",
        PoolGUIDs = new List<Guid>() { decoderPool.GUID },
        FunctionGUID = functionDefinition.GUID,
        FunctionName = functionElement.Name,
        PK = nextEntryData.IndexValue,
        DmaID = functionElement.DmaId,
        ElementID = functionElement.ElementId,
        LinkerTableEntries = new Tuple<int, string>[0],
        MaxConcurrency = 100,
        Capacities = new List<Skyline.DataMiner.Net.SRM.Capacities.MultiResourceCapacity>()
        {
            new Skyline.DataMiner.Net.SRM.Capacities.MultiResourceCapacity()
            {
                CapacityProfileID = capacityParameter.ID,
                Value = new CapacityParameterValue(maxQuantity: 100)
            }
        },
        Capabilities = new List<Skyline.DataMiner.Net.SRM.Capabilities.ResourceCapability>()
        {
            new Skyline.DataMiner.Net.SRM.Capabilities.ResourceCapability()
            {
                CapabilityProfileID = capabilityParameter.ID,
                IsTimeDynamic = false,
                Value = new CapabilityParameterValue(minRange: 1, maxRange: 10),
            }
        },
        Properties = new List<ResourceManagerProperty>()
        {
            new ResourceManagerProperty(Name: "Satellite state", Value: "Down"),
            new ResourceManagerProperty(Name: "Fiber state", Value: "Running"),
        },
    };

    rmHelper.AddOrUpdateResources(new Skyline.DataMiner.Net.Messages.Resource[] { decodeFunctionResource });
}

Hope this helps you further