I am now a US citizen and retired here. I worked in Canada and have an RRSP there which I withdrew funds from last year. Canada withheld 25% taxes. How do I report this?

TurboTax only allows me to enter the net amount withdrawn and amount taxable. Given that I've already paid 25% to Canada, how do I calculate the amount taxable by the US? Or do I declare the gross amount in USD, and then under form 8960 Part II b) then declare the amount of foreign tax paid in USD, and if so, how to I tie the two figures together?