You'll need to obtain an RRSP Contribution Receipt to claim this. If the bank has not issued you one, you may want to check online with CRA. If you sign in to your CRA My Account, check the Tax Information Slips for the year the contribution was made. It just may be on their system. It could also show up as an "Unused RRSP Contribution" with them. If it does, then you can import that information into your 2018 tax return and claim this right away.

If CRA has no record of it, your only option is to obtain an actual RRSP Contribution Receipt and report it by doing an adjustment to the tax year which it applies.