I have created a custom Global Action, and am trying to predefine a value for the Related To field. Related To appears in the picklist of available fields for Predefined Values, so it appears this is possible, but I can't figure out what syntax/formula to use to specify a specific account. Thanks!

Syntax I've tried so far:

(Account(AccountName))
Account(AccountName)
Account.AccountName

And a number of other variations. I've also tried using the ID of the account pulled from the URL.