I submitted my 2018 tax return March 10 . I was unable to add my RRSP Contribution from January 2018 to March 2018 with software. How can i submit after the fact?

Since RRSP contributions don't follow a calendar year, your receipts for contributions made in the first 60 days of 2018 actually belong on your 2017 tax return. You can add these amounts by way of an adjustment. If you're a CRA My Account holder, you can use the "change a return" option from within the portal.

https://turbotax.community.intuit.ca/replies/2647992