SharePoint Server 2013 brings a significant advancement to Workflow: enterprise features such as fully declarative authoring, REST and Service Bus messaging, elastic scalability, and managed service reliability.

SharePoint Server 2013 can use a new workflow service built on the Windows Workflow Foundation components of the .NET Framework 4.5.

This new service is called Workflow Manager and it is designed to play a central role in the organization. Processes are central to any enterprise and workflow is the orchestrator of processes.

When you create a workflow in SharePoint Designer 2013, it provides you with the option of choosing the platform on which you wish to build a workflow in the workflow creation.

Needless to say, SharePoint 2013 style workflows are incredibly amazing because of its Call Web Service action. It is hands-down one of the coolest features as it enables you to build some great solutions.

Ensure that you have configured workflows to be able to run with elevated privileges in your site collection. SharePoint 2013 Workflows has newly introduced the Call HTTP web service activity to implement the REST API calls. Users can create a SharePoint site and can also modify the master page using this activity.

I have documented this process and if you consider yourself on the same boat trying to build this solution – I hope this would surely help!

Click the link to get started – Creating subsites and change master page using REST API from SharePoint Designer Workflow