Follow these steps to confirm a new authorized representative or to accept changes to the authorization level of an existing representative using the new “Confirm my Representative” service.
If your are trying to delegate access to your GST account you must use the confirming process for business and have access to “My Business Account”.
Follow the steps below to confirm the representative using My Account:
* You must have access to My Account. If you haven’t used these services before, you will need to register for an account before authorizing a representative and keep in mind that registration may take several days to be completed.
If you do not have access to your CRA account, your representative can do it for you, but you will need to provide your representative with information from your notice of assessment or from a tax return that was processed at least six months earlier. When your representative submits their authorization request, they will need to provide this information.
But attention, if you choose this option, you may be contacted by the CRA by phone to verify the request, and if you do not answer the phone, the authorization will be canceled and the process will need to be done again.
Source: CRA Website