On the RRSP Profile page check Made RRSP contributions during 2018 or the first 60 days of 2019. Also check Made RRSP/SPP contributions you wish to carry forward and deduct in a future year.
Enter your RRSP contribution.
Click Done with RRSPs
On the RRSP/PRPP and SPP Carry Forward page change the amount in the box for RRSP/PRPP contributions you are deducting for 2018 to zero. This will carry forward the contribution amount to be used in future years.
Please note you can change the amount in this box to any amount up to your full contribution amount. So you can claim a partial amount if that helps with your taxes.