If you know that the T5008 reported transaction will result in a "superficial loss", then you would enter an amount in Box 20 equal to the amount shown in Box 21 so as not to report a capital loss (that is superficial) in this transaction.
TurboTax will not recognize these as Capital Losses because it has no way of knowing that the sale corresponds to an in-kind contribution to a registered account. CRA states that you do not deduct these "superficial losses" when you calculate your income on your tax return.
From TurboTax Blogs:
