Simplest example using UCMA UserEndpoint to send an IM. Note: no error handling, assumes IM always accepted etc. For learning only :)

UCMA UserEndpoint Sending IM Example.cs

using Microsoft.Rtc.Collaboration;
using System;

namespace SimpleUserUCMA
{
    class Program
    {
        private const string sipaddress = "sip:[email protected]";
        private const string username = "USERNAME";
        private const string password = "PASSWORD";
        private const string domain = "DOMAIN";
        private const string destinationSip = "sip:[email protected]";
        private const string IMMessage = "Hello!";

        static CollaborationPlatform _collabPlatform { get; set; }
        static UserEndpoint _endpoint { get; set; }
        static bool _OKToQuit = false;

        
        static void Main(string[] args)
        {
            string userAgent = "ClientPlatformExample";

            var platformSettings = new ClientPlatformSettings(userAgent, Microsoft.Rtc.Signaling.SipTransportType.Tls);
            _collabPlatform = new CollaborationPlatform(platformSettings);

            //Start up the platform, calling back asynchronously once it's done.
            _collabPlatform.BeginStartup(EndCollabPlatformStartup, null);

            //In this example, wait for everything to finish before exiting
            while (!_OKToQuit)
            {
                System.Threading.Thread.Sleep(2000);
            }
        }

        private static void EndCollabPlatformStartup(IAsyncResult ar)
        {
            _collabPlatform.EndStartup(ar);

            //A collaboration plaform can have one or more Endpoints. An Endpoint is tied to a SIP Address.            
            UserEndpointSettings settings = new UserEndpointSettings(sipaddress);
            settings.Credential = new System.Net.NetworkCredential(username, password, domain);
            settings.AutomaticPresencePublicationEnabled = true;

            _endpoint = new UserEndpoint(_collabPlatform, settings);
            _endpoint.BeginEstablish(UserEndpointEstablishCompleted, null);

        }


        private static void UserEndpointEstablishCompleted(IAsyncResult ar)
        {
            _endpoint.EndEstablish(ar);

            //Once the endpoint is in place, create a Conversation and an IM Call. 
            var Conversation = new Conversation(_endpoint);
            var Call = new InstantMessagingCall(Conversation);

            //When the call is established, Flow will be created. Flow is how you sent IMs around. Therefore, just before
            //establishing, we attach an event handler to catch the flow being setup (it's state will change to Active)
            Call.InstantMessagingFlowConfigurationRequested += Call_InstantMessagingFlowConfigurationRequested;
            
            Call.BeginEstablish(destinationSip, new CallEstablishOptions(), EndBeginEstablish, Call);
        }

        private static void EndBeginEstablish(IAsyncResult ar)
        {
            Call call = (Call)ar.AsyncState;
            call.EndEstablish(ar);
        }

        static void Call_InstantMessagingFlowConfigurationRequested(object sender, InstantMessagingFlowConfigurationRequestedEventArgs e)
        {
            //Once we're notified about this, we get a handle to the newly created Flow. Let's use this to register for state changes.
            e.Flow.StateChanged += Flow_StateChanged;
        }

        static void Flow_StateChanged(object sender, MediaFlowStateChangedEventArgs e)
        {
            if (e.State == MediaFlowState.Active)
            {
                //The flow is now active! We can use it to send messages.
                InstantMessagingFlow flow = (InstantMessagingFlow)sender;
                flow.BeginSendInstantMessage(IMMessage, EndBeginSendInstanceMessage, flow);
            }
        }

        private static void EndBeginSendInstanceMessage(IAsyncResult ar)
        {
            InstantMessagingFlow flow = (InstantMessagingFlow)ar.AsyncState;
            flow.EndSendInstantMessage(ar);

            //Having sent the message, terminate the conversation
            flow.Call.Conversation.BeginTerminate(EndBeginTerminate, flow.Call.Conversation);
        }
        
        private static void EndBeginTerminate(IAsyncResult ar)
        {
            Conversation conversation = (Conversation)ar.AsyncState;
            conversation.EndTerminate(ar);
            
            _OKToQuit = true;
        }


    }
}