in essence you want...
laptop off
connect laptop up to cradle
set phone into cradle
computer starts
computer logs in to desktop
sorry.. i'm unaware of any product which does this in exactly the manner you wish. you will still need to physically push the power button to start your pc unless it was in sleep mode in which case it could be set to "wake" from input. in sleep mode you could wake it possibly by connecting the phone up to usb but you would still need to type your password in on the laptop. remember that in many cases sleep mode still draws some power so you would be losing battery charge while on the move and having a sleeping computer in a pc bag isnt a good idea. typically you would be in a shutdown state.
you might want to look into "pc remote control" apps as they might give you some sort of function but i'm not sure if they can actually log in to the pc (not remotely... actually logging in like you wanted). you could look but i wouldnt get too excited just yet.
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personally i'd like to suggest an alternative method.
you would still be required to push the power button still (but honestly how easy is this)... but instead of typing in a password you could...
use a fingerprint biometric to swipe your finger and log in.
use the built in webcam on your laptop to log in via face recognition