Hi there, @Jjhog2019.
Glad to have you here in the Community. Allow me to share some insights about recording a taxable payment in QuickBooks Desktop.
Are you referring to adding amount to the partial payment of your invoice? If so, this option isn't possible. When recording a taxable payment in QuickBooks Desktop the entire invoice amount will have it's entire taxable amount. If you added a partial payment, the taxable amount will be attached to the remaining balance. For example the invoice amount is 100, it has a sales tax for 20. When you added a partial payment for 70 the remaining balance will be 50 since its counted together the tax amount.
However, I'd encourage you sharing your suggestion to our developers. Doing this helps them understand where we can improve and what features should be added so we can better serve you.
In order for them to account your suggestions, please follow these steps on your QuickBooks Desktop account:
Please let me know if you're referring to something else. I'm here to help you out whenever you needed me. Have a great day!