Chrome Extension- Change download folder for specific download
You can't change the base Downloads folder - this setting is not anyhow exposed.
However, with chrome.downloads API you can put files in a subfolder of the user's Downloads folder.
You can either initiate the download yourself with chrome.downloads.download or use chrome.downloads.onDeterminingFilename request to intercept downloads from other sources.
You need to provide a relative path as a suggested name, e.g. "special/filename" to save into a subfolder special inside the Downloads folder.