The easiest way to do it would be to pick an ID in the future (say 20 ahead of the latest one) then type the title and link out, then just keep refreshing the page for your selected ID minus one (for example, if your chosen ID is 3743040 refresh news.ycombinator.com/item?id=3743039) until you get a result then hit submit, if you're not too slow you'll get the ID 3743040.
an ID can refer to a comment or a link, you can find the latest comments: http://news.ycombinator.com/newcomments latest links: http://news.ycombinator.com/newest
The easiest way to do it would be to pick an ID in the future (say 20 ahead of the latest one) then type the title and link out, then just keep refreshing the page for your selected ID minus one (for example, if your chosen ID is 3743040 refresh news.ycombinator.com/item?id=3743039) until you get a result then hit submit, if you're not too slow you'll get the ID 3743040.