With manual sort order, tracks are always added at the end of the playlist. The "Sort Ascending" and "Sort Descending" toggles are unavailable. I propose for them to become functional in manual sorting, with "descending" reversing the playlist and causing all newly added tracks to appear at the top.
Unless I'm missing something, it's not currently possible for the playlist to be configured this way – you can sort by date added in descending order, but that's the date the track was added to the library, not the playlist.