Hi there,
Your business is assigned a Tax ID/business number by the CRA when you first register for GST/HST, payroll deductions, import/export privileges, or corporate income tax. I know how important it is to have this number in order to properly run your business. I'll explain how to find this number, and how to enter it in QuickBooks Online.
This number is assigned to you by the CRA, so you'll have to refer to their website in order to learn how to find your number. Here's a link I found that may help: Business number registration - CRA.
Once you know what the business/tax number is, you can enter it in your QBO account by clicking Taxes > Sales Tax > Manage Sales Tax > click the drop-down menu and click Edit Agency Settings > enter the GST/HST number. This number will appear on your sales forms for reference.
If you have any other questions, or you're still unsure of the proper steps to take, please reach out to our tech support team.
Have a great day.