How to copy values between fields using triggers

I'm currently trying to create a trigger to copy a custom text field on a ticket into a custom user field. When I use the following placeholder as the value to set user field to on the action, it doesn't treat the placeholder as a variable, and instead literally copies this text into the user field:

{{ticket.ticket_field_4971504499091}}

See below for a screenshot of what I'm trying to do: