I am building a SharePoint workflow, I need to get data from a SharePoint list and also from external system by calling RestAPI. Can someone suggest how to do it in SP designer 2013?
Maybe you should create web service. Make custom actions in this service and call from designer workflow. https://msdn.microsoft.com/en-us/library/office/dn567558.aspx
SharePoint 2013 designer has lots of new actions, which provides developers the flexibility to write complex workflows and perform various operations on SharePoint objects.
Some of the new actions are: a. Start a list/site workflow. b. Build Dynamic Value. c. Call HTTP Web Service.
The introduction of the call HTTP Web service is the most effective one, as we call a rest API to fetch all items from SharePoint list and iterate over them. Also, we can create a new item in another list by using the rest API(Post) request.
Here is the video link which will guide you, how to use the SharePoint workflow action “Call HTTP Web Service” and how to iterate the collection of response object from the rest API