Welcome to the Community. Being able to amend a T4 is essential to ensure that the correct information is submitted to the CRA. QuickBooks Online is a great program that offers the ability to pay your employees and track your remittances. I'd be glad to point in the right direction to amend your T4.
If you'd like to add an amount on the T4, You'd need to obtain the T4 form from the CRA to manually make the adjustment on it.
I'd encourage to consult an accountant as they have the knowledge to point you in the right direction on how to amend your T4. If you don't have an accountant, you can find one near you by clicking on My Accountant on the left navigation menu> and thenFind a pro to help.
Let me know if you have any questions, I'll be happy to help.
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