457, or 403(b) plan, US Medicare and Federal Insurance Contributions Act (FICA), you must add these contributions to your foreign employment income on line 10400 of your Canadian return. These contributions may be deductible. See line 20700.
https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/tax-return/completing-a-tax-return/personal-income/line-10400-other-employment-income/line-10400-foreign-employment-income.html