Process builder not supporting “Outbound Messaging” with in out of box functionality, but have workaround to archive that functionality instead of going separate workflow rule send outbound message.
You can archive using Apex Code. Create “@InvocableMethod” method apex code with HTTPS callout. Call that method inside process builder.
Sample Code :
public class OutboundMessaging
public static void sendToExternalSysstem(List<Id> accountIds)
List<Account> lstAccount=[select id, Anme from Account where Id in :accountIds];
//Build HTTP callout here
Use this method inside Process Builder
Note : You can archive this functionality using Workflow Rule. This is out of box functionality.