There is a DOWNLOAD section in the article after section - Adaptive form using XFA form template. It contains link to download the boilerplate package. The name of the boilerplate package zip is Prefill-Sumbit-XMLsAndContentPackage.zip. Kindly take a look. If you still face the issue, use direct link https://helpx.adobe.com/aem-forms/6-2/prepopulate-adaptive-form-fields/_jcr_content/main-p ars/download_section/download-1/file.res/Prefill-Sumbit-XMLsAndContentPackage.zip.
Thanks for your reply. However, the zip you gave here doesn't contain the boilerplate project.
Was the boilerplate zip ever located? I found the zip file provides examples for preloading data via URL. I did not see anything related to an example of a prefill service.
Specifically AdaptiveFormContainer (configuration) => Prefill Service