Json Rest API in my C# winforms application targeting .net 4.8

Hi there

I need help with implementing Json Rest API:
It has to be implemented in my application using .net 4.8 !

There is a action platform https://ricardo.ch
They provide a Rest API but I dont understand these Rest Services.
This is the documentation (quick start):
https://help.ricardo.ch/hc/de/articles/115002970729-How-to-start-quickly
https://ws.ricardo.ch/RicardoApi/documentation/html/N_Ricardo_Contracts.htm
I dont understand how to call it (as I dont enderstand json at all).

What I need to do with this ricardo.ch API:
All I need to be able is, to get all of my sold articles within a period of time.
For example: GetSoldArticles("count of days back from today")

This project is more a helping project, maybe we will need also some TeamViewer sessions
You have to write some code blind because I am not allowed to give the Key/Secret to anybody else.
I think, for somebody who has really good json, .net and C# knowledge, it is an easy task.

Please no agencies
Best regards, Frank Uray