It should look like this:
"startTime": "2019-03-29T23:30:00+01:00"
With default I get
20190430T151332Z
But I cannot figure out how to get the desired syntax. Is that possible with ARM template utcnow function and if yes, how?
you'd need to use standard dotnet custom formatting. As described here.
"[utcnow('yyyy-MM-dd THH:mm:ss zzzz')]"
this seems to be fine, except it returns not your TZ, but ARM TZ, so it always returns +00:00. so it makes no sense to use it.