Using autofill is equivalent to accessing the taxpayers CRA online account. So the taxpayer must authorize you before you use the autofill service.

The taxpayer can authorize you by accessing their own account and authorizing you using your RepID

Or, in Profile you can JUMP to AUTH, complete the authorization form and have the taxpayer sign it. Then in Prife to to the FILE menu and choose EFILE AUTHORIZE A REPRESENTATIVE. Once you have received confirmation that the authorization has been accepted, you can use Autofill

Larry Hancock