You can find the PPPoE Settings at Interfaces → Assign → PPPoE.
PPPoE can only be configured on top of:
- Physical Interfaces
- VLAN Interface
PPPoE is commonly used for DSL dial in connections.
Username is the PPPoE Username
Password is the PPPoE Password
Assign to new Interface is an option shown when you create a new VLAN and it will automatically assign the Interfaces as described in Assign Interfaces.