To enter your Tax Paid by Instalments, in the sidebar of TurboTax Online:

Go to Deductions > Payments Profile > click on the Payments Profile that appears and you will land on the page "Your Payments Profile for 2018". Check the box beside Paid tax instalments for this year, then continue to the page "Paid tax instalments this year". 

(NOTE: If you are using TurboTax Online Free, you cannot click directly on Tax Instalment Payments, or use the Find feature. You must check the box on the Payments Profile page.)