Add created character to Companions tab

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.