vlan asuswrt merlin ac-68u

Thank the author and the repliers of https://www.snbforums.com/threads/vlans-on-merlin-mini-howto.20529/, so that we know to config VLAN on Asuswrt/Merlin. But I need to know how to configure the default VLAN ID of a aggregated port, so that all incoming frames with no tag from this port...