You'll need to create an expense transaction and categorize it as other business expenses. However, I'd still recommend consulting with an accountant so you'll be guided in claiming your self-employed related deductions. Let me show you how to do it.
Go to Transactions, then Add transaction.
Enter the transaction information and then select Other business expenses under Select a category.
Hit Save.
Once you're done, you can then itemized your tax upon importing to TurboTax. Please check this article for more information about the different expense categories in QBSE.