I can help you add the details you need for your purchases in QuickBooks Online. Before we start, allow me to share some insights about your concern.
In the United States, vendors already include the taxes with the cost of the items you purchased. For example, if you bought supplies, you record the total amount paid (including the sales taxes already) to office supplies.
When splitting expenses through bank feeds, please know that adding taxes is still unavailable. If you're required to do so, I suggest creating a separate expense account and record them manually.
Click the Gear icon in the upper-right corner and select Chart of Accounts.
Press the New button in the upper-right part.
Below AccountType, choose Expenses.
Choose the detail type you prefer.
Enter a name of your choice, then hit Save and Close.
Click the + New button and select Bill or Expense.
Below Category details, add the expense account you created.
On this line, enter the amount of sales tax in the Amount column (this will need to be calculated manually).
Under Item details, add the products and services including the amount (ensure that any detail in this transaction adds up to the correct total).
Hit Save and close.
Also, It would still be best to seek expert advice from an accountant to ensure your books will be correct and error-free.
I've included an article you can use in the future to handle sales taxes, like how to set up and track them in the system: Sales tax in QuickBooks Online.
Let me know in the comment section below if you have any other questions or concerns. I'm always available to lend a hand. Have a great day ahead.