Hi,
After searched a while and tested in my task scheduler, the monthly triggers are limited to specific format which we have fewer changes could be customized.
But my concern here is that if the first week started later than chosen Monday (for example), will this week countered as the first or simply calculating from the next week which contains Monday.
Perhaps we could test from this way and I will update once there is any result.
Hope this helps and please help to accept as Answer if the response is useful.
Thanks,
Jenny