I assume this is the commutation tracks on the encoder?
If so, what is the nature of the feedback? Pulse(s)?
In all likely hood it is a AC servo and not BLDC, both motors are essentially identical, just the commutation differs.
Unfortunately I only have the process for BLDC.
This is done by powering the encoder and back driving the motor so it generates, and displaying the feedback and the encoder output on a double beam 'scope.
And turn the encoder until alignment is achieved.
Al.