Thanks for posting in the Community. I'd be happy to answer your question and steer you in the right direction.
At this time, there isn't an option to manually enter GST on purchases. QuickBooks Self-Employed will automatically assign a tax code which matches your province. For any questions or inquiries, I encourage you to reach out to the Self-Employed team using this link here. Once you click the link, make sure to select the Self-Employed support option.