I have a contact object and a custom object called "Product User"
The Contact is the parent and the "Product User" is the child.
I already have Contact data and I am looking for Upserting records in "Product User" using Simple-Salesforce or the Salesforce REST API.
I have a script containing "Product User" fields and since "Id" is required for the Parent Lookup, I have grabbed the "Contact External ID" from Contact object which holds the external unique Contact ID from our web app that we want to use for creating that lookup relationship.
How can we create the lookup relationship by leveraging something like [ObjectName]:[ExternalIDField__c] for the Salesforce API on upsert calls. Ideally, we would then create a net new contact when the external ID does not exist and then reparent after successful insertion from the REST API.