You would enter your T4A slip on the T4A page, then when you go to the self-employment section and create a business, then the T4A amount will automatically be entered.
To enter in a T4A slip in TurboTax Online, click “Find” and search for “T4A”. Then select it from the list and click on the “Go” button.
If you have TurboTax Live Assist & Review, here is how to talk to a Tax Expert: https://turbotax.community.intuit.ca/community/navigating-turbotax/help/i-m-using-turbotax-live-assist-review-how-do-i-talk-to-a-tax-expert/00/873764