You can call the dox42 WebService from a SharePoint Workflow.
Security Advice
To use a SharePoint Workflow with SharePoint on premises to call dox42 allows for a very simple but effective security configuration. You can restrict the access to the dox42 WebService to the IP of your SharePoint Server. That way your dox42 Server can only be called from your SharePoint Server, thus the SharePoint user permissions are enforced.
Steps
Create a Workflow variable e.g.
dox42Request
.Store the dox42 REST link to this Workflow variable. You can use the String Builder to create dynamic parts of the link, e.g. input parameters.
Create a “call HTTP WebService Action” and use the Workflow variable “dox42Request” as WebService URL.
Result
You can now call the dox42 WebService from a SharePoint Workflow.