Custom Action Button Calculated

Hi! I'm trying to create a new Action button for Accounts that relates to Meetings; specifically to create a new Meeting where the meeting subject is: ("TAF" + "name of related Account"). 

TAF is an acronym used internally to designate the purpose of the meeting.

In the Accounts module I have created a Textfield: "taf_value_c" with a default value of "TAF".

Ex. Account name = "Meyer's Grocery Store"

taf_value_c = "TAF"

User clicks button > button opens Meeting create screen with subject "TAF Meyer's Grocery Store"

I am trying to configure the button to concatenate taf_value_c and Account name and have two issues.

I would think this Default value would apply after Edit*save on the account, but the Textfield remains blank until I edit the field itself on the record layout:

 

Below was my button formula, which was accepted but is not working. After pressing the button the subject is empty.

concat(string(related($accounts, "name")), string(related($accounts, "taf_value_c")))

Does anyone see any obvious errors?

Thank you