Instant on mode xbox?

How and where can I activate the mode?

You can switch on the instant-on mode as follows:

Press the Xbox button to open the guide.
Then select System> Settings> General> Power Mode & Start.
From the dropdown menu you can then select instant-on mode or energy-saving mode.

The setting is then applied directly and does not require a manual restart of the console.

You can read more about this on the https://beta.support.xbox.com/help/hardware-network/power/learn-about-power-modes.