I try to connect my A518 OD activating by means of switch

No the NSS is two separate switches in one "box." The two outer pins are one switch, which is for reverse. The center pin is the NSS which is a separate switch, and it goes to ground in park or neutral

What is so hard? About using a relay. EASY. Look up "Bosch Relay"

View attachment 1715659196

Connect 30 to a substantial 12V power, as this runs the solenoid which draws a fair amount of current, use no12 wire and it really should be fused/ breaker

The solenoid connects to the "normally open" (NO) contact, 87

85 is one end of the coil and you run a wire from here to the center pin of your NSS

86 is the coil "hot" end and this goes to one end of your start source, in this case your push button. Other terminal of push button goes to 12V. This can be light duty wire as the coil draws little

87A does not get connected.
Same relay setup I’m using for my fan and fuel. So we may be saying the same thing. I could essentially use the center pin on the NSS as the 85 ground of my relay, giving me a “safety Neutral Switch”, yes?