What is my username in Active Directory?

'whoami' shows that your process is running as a local (non-domain) account. It's not a standard user account but an automatically managed "IIS application pool" account.

Because it's running under a non-AD account, the process only has Active Directory access using the computer's own machine credentials. In this case, MyUsername123$ is the machine's AD account name (note the trailing $). Computer accounts are not shown in 'net user /dom'.

After trying to access an AD resource, use klist to see the Kerberos tickets available to the current process. The "Client" field will tell you exactly what AD account you're using.