Measuring and conditioning ignition coil secondary signal
Measuring: use a capacitive probe with 1000:1 attenuation to measure the signal on the scope. By wrapping around the ignition cable, the clamp of the capacitive probe effectively forms a cylindrical capacitor in the order of a few pF. The attenuation block in the probe most probably completes the circuit with a capacitive divider, similar to what your linked document depicts.
Permanently connecting: you can use the same arrangement as the measurement and build yourself a capacitive clamp and attenuation circuit. Another option is to use inductive coupling. An example is the VW 113-919-105 sensor that was used to detect the spark on cylinder #1 for some vintage Volkswagen vehicles (Type 2 and Type 3). It was a ring in which the HT wire was inserted, which makes it probably more resistant to vibration than a bigger clamp plus attenuator block.