Electromagnetic coupling on printed circuit board traces
I would like to know: How can I confirm that it is a electromagnetic
coupling and how can I protect my circuit from it?

At 400 kHz and with the short length (10 cm) of &quot;antenna&quot; you used, the coupling is not electromagnetic but just plain ordinary electric coupling (one half of EM coupling). Basically, it's capacitive coupling of the 10 kV damped waveform to the input.
Protection methods: -

rail clamping diodes
series resistors
parallel capacitors
parallel TVS diodes
a buffer circuit

Are the obvious ones to to try but, they might also &quot;ruin&quot; the signal that is on that line so you need to be a little more open about what that line is used for and what signals are on that line.