I want shutdown but NOT turn off...

The only guaranteed way to do what you wish is to physically circumvent the Soft start switch used on modern motherboards.

The front mounted power switch is a momentary action switch that sends a pulse to the motherboard to either instigate a wake-up or shutdown sequence, depending on the current state of the system, (the long press to instigate a power break for problem situations is also monitored for).

It may be possible to replace or bridge the front mounted power switch to give an always on supply to the motherboard. Of course this may have the result of triggering a shutdown immediately after start-up as a result a constant on signal - this would need to be tested on the particular motherboard as the whether it monitors for a change of state or just an on state of a specific period.

You may also be able to procure suitable server motherboards that has a straight On/Off power supply system to work better unattended with a UPS

Barman58 said:

The only guaranteed way to do what you wish is to physically circumvent the Soft start switch used on modern motherboards.

The front mounted power switch is a momentary action switch that sends a pulse to the motherboard to either instigate a wake-up or shutdown sequence, depending on the current state of the system, (the long press to instigate a power break for problem situations is also monitored for).

It may be possible to replace or bridge the front mounted power switch to give an always on supply to the motherboard. Of course this may have the result of triggering a shutdown immediately after start-up as a result a constant on signal - this would need to be tested on the particular motherboard as the whether it monitors for a change of state or just an on state of a specific period.

You may also be able to procure suitable server motherboards that has a straight On/Off power supply system to work better unattended with a UPS

When start button jams, computer just keeps on cycling on and off. A simple IR or EF relay could also do the trick.

It may need to have some re-wiring - replacing the feed from the motherboard supply relay to the system power with a direct feed from the PSU which would circumvent the front panel switch and the related circuitry on the motherboard

Could use something like this to make your low power (Start) switch.
http://www.mikroprinc.com/proizvodi/...jnik/index.php
or something like this: Elektronika i komponente : Prekidač 12V/10A sa RF daljinskim 433MHz relej 27.02.2017 - ID 38902901 - KupujemProdajem