That doesn't make sense (the help desk response).  A CPP retroactive payment is not deducted in the current year.  It is included in taxable income, but qualifies for a special tax calculation.  CRA calculates the tax on the income as if it had been included in the prior year(s) and credits the current year's taxes if they would have been less.  This is something that is done automatically by the CRA and cannot be "claimed" by the taxpayer.  There is a Form T1198 in Profile which I have never used but appears to be a form where you can report the amount of the retroactive payment, but supposedly CRA already knows that amount.  The Form T2203 you mention is for if there are multiple jurisdictions and other questions in this community indicate that there are some issues with that form.  Maybe you could just override the amount on the Form T2203 (top line) so that it agrees with the income on line 23600 of the T1?