Tribe or nation name should be used in the LNAB field for Native Americans who did not have a surname. The Canada First Nations project has similar guidance.
Here is the guidance from the Native Americans project:
Name Field Guidelines: WikiTree's underlying technology requires an entry in the "Last Name at Birth" field. It is the policy of WikiTree's Native Americans project to use this field to record the tribe or nation name for those individuals born before the introduction of surnames. For example, Pocahontas was a member of the Powhatan people. Her profile is Powhatan-3.