Box 135 - Private Health Services Premiums is found on your T4A slip. You can enter this tax slip by going to Income > T-Slips > T4A - Pension, Retirement, Annuity.
After entering the amount in Box 135, it will be transferred to your Medical Expense Profile and be added to any other medical expenses you may be claiming.
I'm running TurboTax Standard 2023 (v00.01.61 Build 6004 068G22) and the "T4A slip > Click here to enter Other boxes" option appears to have omitted the "Box 135 (Recipient-paid premiums for private health services plans)" entry.
Please forgive this question, as I'm obviously having an issue with my eyesight! I realized now that I was searching for the field in the "Click here to enter Other boxes" sub-panel.
My humble apologies and thanks for the assistance!
