These keywords are hardcoded by Intuit.  You can, therefore, use only those made available by Intuit.

Currently, my understanding is that only the following are allowed:

$client_name$
This will pull up the taxpayer's first name.

$firm_name$
This will pull up your firm's name.

Intuit is not the quickest to update their articles or known for customer communication.  In case there is anything you need urgently, you could post your question here and someone may be able to give you some pointers or share with you their experience.