RRAS NAT with PPPoE

OS: Windows Server 2016

I'm trying to use RRAS's demand-dial feature for NAT, so that computers on the LAN are routed through a PPPoE connection. When using the wizard, it asks for the credentials for the PPPoE, including username, domain, and password. What is this "domain" field for? I've tried leaving it blank, but get an error about "credentials invalid or protocol not supported". If I create a PPPoE adapter in the "Network Connections" panel (general Windows connections, unrelated to RRAS) using the same username/password (it doesn't ask for "domain"), it connects just fine. Any ideas of why it would work fine there, but not in the RRAS NAT?

EDIT: For clarification, I know what a domain is, but I'm not sure which domain it's referring to here.