csomStartSiteWF.cs

using Microsoft.SharePoint.Client;
using Microsoft.SharePoint.Client.WorkflowServices;
using System.Collections.Generic;
using System.Linq;
using System.Security;

namespace CSOMWorkflow
{
    class Program
    {
        static void Main(string[] args)
        {	
	    //Replace the following values according to your configuration.
	    
	    //Site details
            string siteUrl = "https://mytenant.sharepoint.com/sites/test";
            string userName = "user@mytenant.onmicrosoft.com";
            string password = "password";
            
            //Name of the SharePoint2013 Site Workflow
            string workflowName = "SharePoint2013SiteWorkflow";

            using (ClientContext clientContext = new ClientContext(siteUrl))
            {
                SecureString securePassword = new SecureString();

                foreach (char c in password.ToCharArray()) securePassword.AppendChar(c);

                clientContext.Credentials = new SharePointOnlineCredentials(userName, securePassword);

                Web web = clientContext.Web;

                //Workflow Services Manager which will handle all the workflow interaction.
                WorkflowServicesManager wfServicesManager = new WorkflowServicesManager(clientContext, web);

                //The Subscription service is used to get all the Associations currently on the SPSite
                WorkflowSubscriptionService wfSubscriptionService = wfServicesManager.GetWorkflowSubscriptionService();

                //All the subscriptions (associations)
                WorkflowSubscriptionCollection wfSubscriptions = wfSubscriptionService.EnumerateSubscriptions();

                //Load only the subscription (association) which we want. You can also get a subscription by definition id.
                clientContext.Load(wfSubscriptions, wfSubs => wfSubs.Where(wfSub => wfSub.Name == workflowName));

                clientContext.ExecuteQuery();

                //Get the subscription.
                WorkflowSubscription wfSubscription = wfSubscriptions.First();

                //The Instance Service is used to start workflows and create instances.
                WorkflowInstanceService wfInstanceService = wfServicesManager.GetWorkflowInstanceService();

                //Any custom parameters you want to send to the workflow.
                var initiationData = new Dictionary<string, object>();

                wfInstanceService.StartWorkflow(wfSubscription, initiationData);

                clientContext.ExecuteQuery();
            }
        }
    }
}