Community Text - First Name Text Key

In Studio > Text Editor > Community Text I want to update the text-key 

general.welcome = Welcome back, {0}!

To something like:

general.welcome = Welcome back, ${profile.name_first}!

But I don't know the text-key for the First Name in a user's Personal Information setting.

Currently we are using

general.welcome = Welcome back, {0}!

And {0} is their username. We would like to personalize the message with their first name instead of their username.

Does anyone know what text-key I can use to pull a users first name from their personal information / settings / profile?

Thank you!
Dave