Workflow rule and an outbound message or Process Builder and Apex Code are two ways to integrate Salesforce with an external web service. Workflow rule can define criteria and actions for triggering an outbound message, which is an XML message sent to a designated endpoint URL. Process Builder can invoke Apex Code, which is a programming language that can make callouts to external web services using HTTP requests. Flow Builder and an outbound message or Process Builder and an outbound message are not possible combinations.