In order to add Windows Server/VM as Veeam Proxy or Backup Repository if Server/VM is on Workgroup (This is best practice to isolate Veeam Proxy or Backup Repo from same domain to prevent malware/ransomware/virus attacked if domain admin is compromised)

Go to Run, type regedit

Go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System

Create a DWORD value called LocalAccountTokenFilterPolicy and assign it a value of 1

Restart Server/VM

Done!