Salesforce - how pass field value from Unbounce that is a Look-up field in SF


I would like to link a lead to the referrer (person’s name) meaning, pass form data to Salesforce with a hidden field containing Referrer Name.

Today our system has referers set up as contacts with type = referrer.

When I fill in our Unbounce form with Lead info, and add the referrer name in hidden field called referrer, the integration fails.

The error message is
id value of incorrect type: “persons name here”

why can’'t I pass values into SF that are part of a look up list?
are there any options to add the referrer names to the Lead?