Efiling e-auth(T1013) to CRA is a side function of the myTaxExpress EFile software. You need to have CRA EFiler ID/Password first, and set them in software preference by menu "Start | Preference", then "Tax PreParer Information" tab (Version 2019 and prior) or "Preferences" icon on the icon bar (Version 2020 and later).
Next, you need to create an empty tax return for the client. The client's name and S.I.N information must be correct. Since e-auth doesn't need a birthdate, marital status, and tax province information, you can choose anything when creating such an empty tax return. The tax return is just a place to hold a form of Authorize a Representative (former T1013).
Next, you need to add a form "Authorize a Representative" into this return by menu "Forms | add a tax form". Complete all the required information inside "Authorize a Representative".
Last, you submit the completed form to CRA by choose menu "Start | Authorize a Representative".
Last note, Filing E-Auth will not contain any T1 tax return information to CRA.