Confirmation page as entity autocomplete

Problem/Motivation

I want the confirmation page for mailchimp to be entity_autocomplete instead of a standard textfield.
Much easier for the end user to set it up.

Proposed resolution

Make confirmation page field entity_autocomplete

Current solution

Confirmation page field is changed to entity_autocomplete.
I couldn't figure out how to save this as an actual entity reference so the current workaround is to save the node id as a string.