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This library can help you with fetching flex queries from Interactive Brokers.
Simple call the library by passing your token and queryId:
... FlexResult result = new Reader().GetByApi(token, queryId).Result; ...
Or pass an already downloaded file:
... FlexQueryResponse result = new Reader().GetByString(content); ...
This is the repo that creates the site you are looking at currently. So if you are interested in the technical background, this is the project you were looking for.