field map variable for Load Actian Vector node

I am trying to build a looping workflow where data is lodaded into multiple tables, and table names and other parameters are controlled by variables.

I came against a brick wall trying to supply a variable for the  Actian Dataflow node called 'Load Actian Vector'.

The item is fieldMap, it expects a string variable of some sort. But nowhere in documentation could I find the specs or sytax for it, as it is supposed to be some kind of key:value pairs thing. Its job is to map source and destination table fields.

When I tried various things, I got an error message "Unable load field mapping from json", so I assume that it expects a JSON.

This page from Actian documentation describes similar operation using RushScript, where the parameter is in obvious JSON fromat:   

But when I tried that exact format, I got the same error.

Can anyone help with this?

Do you have a workflow you could send me? I'd like to see your logic.

I am working on a looping sample for a customer now, so I may have somthing I could send you early next week.