Not sure how you were looking to make that trigger.
As you know, there is no separate 'external trigger' input on the automotive scope but you can trigger on any channel.
You also have lots of possibilities on what kind of trigger you want and the thresholds.
This means that you could use something as simple as a push button if you are looking at a physical trigger.
Please feel free to give some more details and we will try to advise.
Catching a stationary screenshot is one thing, analyzing is another thing but catching the signal when it occurs 1 in a 1000 times is more difficult, especially when driving. I already thought about making an external button with 12V but you have to use a channel you cannot use with a useful signal.
I could imagine a external trigger function, an J2534 connection (ELM/Bluetooth) with trigger on signal (like NHV test) or with a keyboard button to mark the position when you feel the engine stall for a second.
To use the scope in diagnosing one of the options would help a lot.
A stationary signal you can catch with every single scope. These advanced options would make the scope extra useful for diagnosing when driving.
catching the signal when it occurs 1 in a 1000 times is more difficult, especially when driving.
It's not difficult at all. Just set the scope up in a continuous streaming mode. One screen with lot's of time continuously scrolling. You do not have to even look at it while driving. When you feel it, just reach over and hit the space bar (after enabling this as start/stop). When done properly, you have it. Guaranteed. If you don't see it in the capture, the monitored signals were not part of the problem.