Private key to WIF process
Firstly, you should never enter a Bitcoin Private Key on an online service (unless you're okay with never owning those Bitcoins again).
Secondly, almost all Private Keys are already in WIF format when you see them. Unless you literally generated a random large number, you probably were not given a raw PrivateKey.
Thirdly, If the data you have is really a PrivateKey, you should be able to import it to BitcoinCore with the importprivkey function. This will allow your bitcoin client to send and sign for the bitcoins you have. You do not need a BIP38 password or anything else. BIP38 Passphrases are used to generate the private key in the first place.