Turn on microcontroller by sensor, auto power off - how?
It ought to be sufficient to connect your sensor signal to the junction of the 220k and the 10k resistors.

All you need is a high signal to cause the whole thing to switch on.



I hope you realize that the very first thing your processor has to do is to set a high signal on pin 2 of the connector.  You also have to do this faster than the length of the sensor pulse.

If the sensor only goes high for 1 millisecond, then your processor has to switch its output high before that 1 millisecond expires.

You must maintain that high state until the processor finishes its task.  When you switch it low again, this circuit will shutoff power to your processor.

By itself, this circuit isn't a latch.  It lets your processor operate it as a latch, but it isn't a latch by itself.