CryptoTrader.Tax only needs "Read" or "View" access to import your trade history. This permission protects your accounts. Learn more about API access in this article here.
Login to your Bittrex account.
Select Account in the top right corner
Select API keys from the menu on the left.
Select Add new key and make sure Read info is toggled and press Save.
Make sure to copy the secret into a safe place, you will not be able to recover it.
Navigate to the Bittrex tab inside CryptoTrader.Tax and enter your API Key and API Secret into the the provided text boxes.
Finally click Sync Transactions to import your transaction history.