Currently, Custom Code is in Beta within our workflows tools. ( More information here)
While this is in Beta, we will have limited documentation regarding this tool but let's start with what we have.
This document here describes the process for setting up a custom workflow action. See our the endpoint tab for detailed descriptions of all the available endpoints.
Creating a custom workflow action makes it easy for customers to integrate your service with HubSpot workflows. First, you define your custom action, including the expected inputs (which must be filled out by the workflow user) and the URL that will be requested when the custom action is executed. Then, customers install your application and add your custom action to their workflows. When those workflows execute, they will send HTTPS requests to the configured URL with the payload that you configured."