MS Dynamics 365 - Create an email activity using dynamics crm Rest API - C#

Prerequisites:

Please refer the below link, to understand how to get azure auth token/ Access token.

https://ankaprasad.wordpress.com/2020/01/08/consume-ms-dynamics-crm-web-api-using-azure-ad-auth-token/

Create email activity in CRM

        CreateEmailActivity("ef9b833b-0000-0000-0000-000000000000", authToken)

        public static void CreateEmailActivity(string recordId, string accessToken)
        {
            JObject email = new JObject();

            email["subject"] = "Email Subject";
            email["description"] = "This is email body";
            email["regardingobjectid_incident@odata.bind"] = "/incidents(" + recordId + ")";
            
            JArray parties = new JArray();

            JObject from = new JObject();
            from["partyid_systemuser@odata.bind"] = "/systemusers(00000000-0000-0000-0000-000000000000)";
            from["participationtypemask"] = 1;  // From Email
            parties.Add(from);

            // Send Email to CRM Contact
            JObject toContact = new JObject();
            toContact["partyid_contact@odata.bind"] = "/contacts(00000000-0000-0000-0000-000000000000)";
            toContact["participationtypemask"] = 2;   // To Email
            parties.Add(toContact);

            // Send Email to email which is not present in CRM
            JObject to = new JObject();
            to["addressused"] = "XXXXXX@gmail.com";
            to["participationtypemask"] = 2;    // To Email
            parties.Add(to);

            email["email_activity_parties"] = parties;

            string emailUrl = Task.Run(async () => await CreateRecord(accessToken, "emails", email)).Result;

        }

        public static async Task<string> CreateRecord(string accesToken, string entityName, JObject entityObject)
        {
            
            string apiUrl = "https://xyka12.api.crm8.dynamics.com/api/data/v9.1/";
            string recordUri = "";
            HttpClient client = new HttpClient();
            client.Timeout = TimeSpan.FromSeconds(360);
            client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", accesToken);
            client.BaseAddress = new Uri(apiUrl);
            client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
            client.DefaultRequestHeaders.Add("OData-MaxVersion", "4.0");
            client.DefaultRequestHeaders.Add("OData-Version", "4.0");
            client.DefaultRequestHeaders.Add("Cache-Control", "no-cache");

            HttpRequestMessage createHttpRequest = new HttpRequestMessage(HttpMethod.Post, apiUrl + entityName);
            createHttpRequest.Content = new StringContent(entityObject.ToString(), Encoding.UTF8, "application/json");
            var response = await client.SendAsync(createHttpRequest);
            response.EnsureSuccessStatusCode();

            if (response.StatusCode == HttpStatusCode.NoContent)
            {
                recordUri = response.Headers.GetValues("OData-EntityId").FirstOrDefault();
                Console.WriteLine("Record URI: {0}", recordUri);
                Console.WriteLine("Record created in MS Dynamics.");              
            }
            return recordUri;
         }

Hope this helps...!!