If you have borrowed money to invest – then you are entitled to a deduction for the amount that you paid in 2018. You need to be able to prove that you have borrowed the money to invest and you have to have a receipt for the amount that you paid. Once you have both of these things covered, you can make the claim on your personal tax return.
An example of this is putting a mortgage on your house and investing in the stock market. If you have borrowed any money to invest you should investigate claiming the deduction. You will find the deduction on line 221 on your personal income tax return.