On the T4A entry page, please click on the blue box next to "Enter here CERB boxes 197 to 205". This will pop up a new window with Box 205 (Seniors' one-time payment) at the bottom. You can enter your amount there.
If you need to get back to that page, you can do so by clicking the "Forms" icon on the blue menu bar. Then select "T4A - Statement of Pension, Retirement, Annuity, and Other Income" from the list and click the "Go" button.
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