Search using custom field in GET call

Content

Hi,

We are trying to create an incident from an external source using the REST API. Reading through the documentation, I believe we would need to use either the Contact ID or LookupName as primaryContact information.

Unfortunately, we do not pass Contact ID through to this external source, and LookupName is unlikely to be unique. We do however have a contact custom field where the value will be unique for each record.

Given that, is there a way to use a custom field as a search option in a GET call? 

I'm currently testing in Postman and I've tried using