This may differ according to your browser, app, or if you have a Mac, but look for the time that your comment posted on Twitter or Facebook. For me, the time is directly under my Facebook/Twitter name. If I double-click on that time, the comment opens in its own window. The URL from that window is what you need for entering giveaways, or if you want to direct someone to a specific comment. You can copy it from your browser bar and paste it where needed.