To Create Call External Web Service in SharePoint 2013 App.  we need to WebProxy class to execute web Service Request in CSOM.  Let Create external Web service in SharePoint 2013  Sample App. remember SharePoint App Developer environment should configured.

How to: Set up an on-premises development environment for apps for SharePoint

How to: Set up an environment for developing apps for SharePoint on Office 365

You can also  use Office 365 SharePoint Online.

Open Visual Studio and Create SharePoint 2013 Project.

Add your Test or developer  Site URL

Open App.Js File from solution Explorer. Cope the Following from Sample code and Paste into App.js

To Call External URL from App you need to set Permission for that URL in you App Manifest

Add your External Web Service  URL in Side tag in my case i am using free online sample of northwind Database which avaliable on services.odata.org domain name.

Open your default.aspx file in App and SP.runtime.js  and Also Executed SharePOint Ready method  

Define html Container with ID in my case i used message in js code so i defined tab with id

 and Hit F5

Download code Link :http://code.msdn.microsoft.com/SharePoint-Call-External-83b49a85

Related Content

• SP.WebProxy object
• Work with data in SharePoint 2013
• Apps for SharePoint overview
• Hosting options for apps for SharePoint
• Build apps for SharePoint
• How to: Create a basic SharePoint-hosted app (A Must)

 By usama wahab khan

Thanks to Microsoft Certified Trainer Team for Welcome MCT Welcome kit.

Javascript :)  

   // ---------- Start workflow ---------- 
    function StartWorkflow() { 
      var errorMessage = "An error occured when starting the workflow."; 
      var subscriptionId = "", itemId = "", redirectUrl = ""; 
 
      var urlParams = GetUrlParams(); 
      if (urlParams) { 
        //itemGuid = urlParams["ItemGuid"]; 
        itemId = urlParams["ID"]; 
        redirectUrl = urlParams["Source"]; 
        subscriptionId = urlParams["TemplateID"]; 
      } 
 
      if (subscriptionId == null || subscriptionId == "") { 
        // Cannot load the workflow subscription without a subscriptionId, so workflow cannot be started. 
        alert(errorMessage + "  Could not find the workflow subscription id."); 
        RedirFromInitForm(redirectUrl); 
      } 
      else { 
        // Set workflow in-arguments/initiation parameters 
        var wfParams = new Object(); 
 
        // get reviewer loginname 
        var html = $("#ctl00_PlaceHolderMain_docReviewerUser_upLevelDiv"); 
        wfParams['DocReviewerLoginName'] = $("#divEntityData", html).attr("key"); 
        // get editor loginname 
        var html = $("#ctl00_PlaceHolderMain_docEditorUser_upLevelDiv"); 
        wfParams['DocEditorLoginName'] = $("#divEntityData", html).attr("key"); 
 
        // Get workflow subscription and then start the workflow 
        var context = SP.ClientContext.get_current(); 
        var wfManager = SP.WorkflowServices.WorkflowServicesManager.newObject(context, context.get_web()); 
        var wfDeployService = wfManager.getWorkflowDeploymentService(); 
        var subscriptionService = wfManager.getWorkflowSubscriptionService(); 
 
        context.load(subscriptionService); 
        context.executeQueryAsync( 
 
            function (sender, args) { // Success 
              var subscription = null; 
              // Load the workflow subscription 
              if (subscriptionId) 
                subscription = subscriptionService.getSubscription(subscriptionId); 
              if (subscription) { 
                if (itemId != null && itemId != "") { 
                  // Start list workflow 
                  wfManager.getWorkflowInstanceService().startWorkflowOnListItem(subscription, itemId, wfParams); 
                } 
                else { 
                  // Start site workflow 
                  wfManager.getWorkflowInstanceService().startWorkflow(subscription, wfParams); 
                } 
                context.executeQueryAsync( 
                    function (sender, args) { 
                      // Success 
                      RedirFromInitForm(redirectUrl); 
                    }, 
                    function (sender, args) { 
                      // Error 
                      alert(errorMessage + "  " + args.get_message()); 
                      RedirFromInitForm(redirectUrl); 
                    } 
                ) 
              } 
              else { 
                // Failed to load the workflow subscription, so workflow cannot be started. 
                alert(errorMessage + "  Could not load the workflow subscription."); 
                RedirFromInitForm(redirectUrl); 
              } 
            }, 
            function (sender, args) { // Error 
              alert(errorMessage + "  " + args.get_message()); 
              RedirFromInitForm(redirectUrl); 
            } 
        ) 
      } 
    } 
 
    // ---------- Redirect from page ---------- 
    function RedirFromInitForm(redirectUrl) { 
      window.location = redirectUrl; 
    } 
 
    // ---------- Returns an associative array (object) of URL params ---------- 
    function GetUrlParams() { 
      var urlParams = null; 
      if (urlParams == null) { 
        urlParams = {}; 
        var parts = window.location.href.replace(/[?&]+([^=&]+)=([^&]*)/gi, function (m, key, value) { 
          urlParams[key] = decodeURIComponent(value); 
        }); 
      } 
      return urlParams; 
    } 

C#

var WSM = new WorkflowServicesManager(web);
var wfSubscriptionService = WSM .GetWorkflowSubscriptionService();
//get all workflows associated with the list
var subscriptions =  wfSubscriptionService .EnumerateSubscriptionsByList(listId);
//run all workflows associated with the list
foreach (var workflowSubscription in subscriptions)
{    
   //initiation parameters    
   var inputParameters = new Dictionary();    
   inputParameters.Add("WFProperty", "Value"); 
   WSM .GetWorkflowInstanceService().StartWorkflowOnListItem(workflowSubscription, itemId, inputParameters);
}

Start: Set up and configure SharePoint 2013 Workflow Manager

Reference :

http://code.msdn.microsoft.com/office/SharePoint-2013-Approval-f5ac5eb2