thank you! I thought something like that. A periodic trigger does not work here since this is a very time-sensitive thing. I would've to set it to something like "every five seconds" which could then be hard on the CPU i guess.
Anyways, I found another (quite ugly?) way to accomplish that: there is a group which gets active in Chrome and the window title matches a specific pattern. (The target URL has a fixed window title that changes to sth completely different once the trigger gets activated, so we're not in a loophole here. And of course the target window/tab must be active for that.) The actual macro triggers when the focused window's title changes. It will execute an IF THEN action: IF the URL contains this certain piece of string THEN execute the macro. The OR section is just empty.
ATM this works for me and that's fine. Nonetheless, I'm a quite unexperienced user of KM but even for me this looks really bad. Is my solution a CPU intensive one? If someone has a nicer solution I'd be happy to here about it!