forge-boilers.nodejs | DEPRECATED , please use https | Runtime Evironment library

A little bit confused with your question. To use the offline example as you mentioned, you first have to convert your original file to SVF by our Model Derivative API "POST https://developer.api.autodesk.com/modelderivative/v2/designdata/job", you check the link https://developer.autodesk.com/en/docs/model-derivative/v2/reference/http/job-POST/ about this API.

When the original file is successfully converted to SVF, you can download all the derivatives including the thumbnail images to your local machine, and then setup the offline viewer example as you mentioned at https://github.com/Autodesk-Forge/forge-boilers.nodejs/tree/master/1%20-%20viewer-offline. About how to download these derivatives, the project 5 & 6 of this github sample can do this. Also, this website https://extract.autodesk.io/ is another easy way to extract the bubbles together with a viewer.

Back to your question about how to get thumbnail images from private data, if you already use our sample to download these derivatives/bubbles, usually the thumbnail images should be already included, if they are not, you can still use this API (GET https://developer.api.autodesk.com/modelderivative/v2/designdata/:urn/manifest/:derivativeUrn) to download it by yourself, you can refer https://developer.autodesk.com/en/docs/model-derivative/v2/reference/http/urn-manifest-derivativeurn-GET/ for more information.

Hopefully, I answered your question:)