I created a custom character to use as an NPC in a campaign I am creating (my first campaign!).

In the maps view for Token browser it has a Companions option.

That says to Add and manage companions through the Extras tab on the character sheet for them to appear here.

I don't see an Extras tab on the character sheet, what am I missing.

TIA

Firstly, you need to create them as a Monster, not a player character sheet.  Then you go to your character sheet, and you should see Extras after Notes, and click Manage Extras.

We usually just create them as characters either unassigned or assigned to the dm.