%LOGONSERVER% is a useful environment variable to use in logon scripts to see which DC has serviced your request and can be handy to reference if you want to access additional files/shares on the DC. Unfortunately this environment variable is only accessible after logon and isn’t useful for computer startup scripts.
When I need to access the DC that’s providing me with GPOs during a computer startup script I emulate %LOGONSERVER% with the following code:
for /f "tokens=1 delims=\" %%i in ('@echo %0') do set DOMCTLR=\\%%i
%DOMCTLR% can now be used in the same way that
%LOGONSERVER% is used.