When a customer emails in, their email subject becomes the ticket title. I'd like to copy that Ticket Subject into a custom field when the ticket is created. I've tried writing “Subject” and {ticket.subject} but neither reference the ticket subject. Picture included:

You can utilize a webhook to set the ticket's subject into the custom ticket field.
Here’s an example for your reference:
Test trigger.
The result:
I hope it helps.
Regards
Aleksandrs