The Galaxy S8 and S8+ haven't been in the hands of consumers for long, but as with almost any Android phone, developers are eager to flash custom recoveries and ROMs onto it. As such, it should come as little surprise that TWRP is already available for the Exynos Galaxy S8 and S8+.
The S8 and S8+ use dm-verity, which means that swiping to allow system modifications at startup will prevent the stock ROM from booting (unless you are using a custom kernel). To bypass this, you will have to first install a custom kernel that has dm-verity disabled in the fstab.
There are a few different ways to flash TWRP to the S8/S8+. If you don't have root, you'll have to use Samsung's Odin tool. If you are rooted, you can use the dd command in a terminal emulator/adb shell or install the TWRP app from the Play Store.
Once you get TWRP installed using any of those methods, you can update it through the TWRP app without root. See the source links below for more details and the recovery downloads.