Assuming your servo amp has a fault output, I would use motion.spindle.N.amp-fault-in.
linuxcnc.org/docs/devel/html/man/man9/mo....html#SPINDLE%20PINS
You will likely need to check and perhaps edit how 'fault' is defined in your drive and make sure that definition covers everything you consider a fault. Like over-current, following error, etc.
Keep in mind that halui pins don't work while the machine in running a program (auto mode), so halui.estop.activate won't work like I think you want.
Bonus: if you have other components you want to trigger an estop (or similar fault condition), a somewhat recent addition to LCnC are
motion.misc-error-NN pins. These are defined in the same HAL line as your digital-in/analog-in values. Thusly:
num_dio=[EMCMOT](DIO) num_aio=[EMCMOT](AIO)
num_misc_error=[EMCMOT](MISC_ERRORS)