PSEXEC -s -i ssms.exe
EXEC sys.sp_addsrvrolemember @loginame = N'CHNET\ADMINUser', @rolename = N'sysadmin';
OR
PSEXEC -s -i \\server cmd.exe
osql -E -Q "EXEC sys.sp_addsrvrolemember @loginame = N'CHNET\AdminUser', @rolename = N'sysadmin';"
.
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Use PSTools' PSEXEC with the -s -i option to run SSMS and you'll have "NT Authority\SYSTEM access"