Extensions cannot save files outside the "Downloads" folder automatically. You can however save files in a sub-folder (see the option "file name > template" > help > third example). An alternative is to install "SingleFile companion" but it requires some technical knowledge, see https://github.com/gildas-lormeau/SingleFile/tree/master/companion for more information.

Edit: Thank you for the feedback! I decided to include the folder name in the template because it can work with other destinations. I also try to do my best within the technical limitations imposed by the extension environment.