I have found a solution.
I drop the Json object and I create a Class Library with my master/details structure in Visual Studio.
The Visual Studio project needs the .NET framework 4.6.
After coding and building the project it’s necessary to create the nuget package, for a reason I don’t know (I didn’t go deeper to understand) the publish option is not available for this version of .NET so it’s necessary to generate the nuget package by hand.
First generate the nuget xml file with
and adapt the xml
After that generate the nugget package using
And finally is just necessary to import inside UiPath the nuget package.
The final touch is in Create Form Task pass the variable of the new type and the result is a table with the fields needed.