You spawn at an airport with active ATIS, Ground and Tower frequencies. When you spawn, you are put on the ATIS frequency by default so you can gather all of the weather and operational information at the airport.
When you switch from ATIS to the ground frequency, there is no need to communicate ‘Standby’ to the ground controller. Controllers know you are there as you appear on their frequency strip. All you need to do is request push back/taxi when you are ready.
Same goes when you switch from ground to tower frequencies. When you are first in line for takeoff, there is no need to communicate ‘Standby’ before requesting takeoff.
See you in the skies!