Trigger to add User custom field data to Ticket custom field

I have a custom User field name Oilsoft ID. I have a custom Ticket field named Oilsoft ID. I want to inherit the User Oilsoft ID into the the ticket Oilsoft ID upon ticket update if there is not already a Ticket Oilsoft ID present.

I have tried to do this with a trigger as follows:

Meet ALL conditions:

Ticket>Ticket      Is   Updated

Ticket>Oilsoft ID   NOT present

Actions:

Ticket>Oilsoft Id    {{ticket.requester.custom_fields.oilsoft_id}}

But the Action simply put the the literal “{{ticket.requester.custom_fields.oilsoft_id}}” in the Ticket Oilsoft ID field.

This seems to indicate that placeholders can't be used here.

So how else could I achieve this functionality?