If that's a secret key then it's not necessary to generate a QR code.
1. Simply download and install Google Authenticator from App store/Play store
2. Open and just click the plus button, it will give an option of either Scan a QR code or Enter a provided key
3. Choose Enter a provided key, type your key
4. Once the key was added there will be a label, so you'll know if it's for your Bittrex account or not
I will also advice to use Authy instead of Google authenticator as this particular authenticator can help you backup your 2FA keys for all your added account fully so you can easily gain access to everything back should in case you misplace your device. It was helpful for me when I misplaced my own device.
This is a good option too (or maybe better) . I didn't know that Authy also allows entering the key manually that's why I didn't suggest it