You really shouldn't type your private key anywhere online, as you are practically giving the website your bitcoins. Instead download Bitcoin Core, you don't have to wait for it to sync up, just go to Help->Debug window->Console and type the
importprivkey "<yourprivatekey>
that should add the standard Bitcoin address corresponding to that private key to the wallet, then put that address in the following command
addwitnessaddress "<addressyouimported>"
. The result of that command will be your segwit address.