If you are using C#, have you tried using the NuGet DocuWare API packages "DocuWarePlatformAPI"? More on that can be found here:
Using that package in .NET is by far the easiest way to utilize the RESTful API (Platform SDK).
Can you try that methodology and see if you can get connected and get a list of file cabinets back? That would mean you are up and running and just need to explore the DocuWare objects presented in the API package until you find how to do whatever you need doing.