Once your business has a CRA business number, you can apply for the GST/HST number.
Sole Proprietorships need to call the CRA at 1-800-959-5525 to register for the GST/HST number, as well as other CRA business program accounts (import/export, payroll, etc).
Corporations can register for the GST/HST directly in Ownr in the “Organization” tab of the Ownr dashboard. Additional CRA business program accounts can be registered at the same time (import/export, payroll) at no additional cost. This service is $50 for companies on the online minute book plan, or free for companies on the managed corporation plan.
The processing time for registering CRA business programs through Ownr is 1-2 business days.
Alternatively, you can contact the CRA directly to register at 1-800-959-5525.
See this guide for further details.