External Content write to SQL DB value assignment not seeing Primary Key

I created an external content type in SharePoint Designer 2013 for SharePoint 2016 on premises using Impersonate Custom Identity with a SQL login using the Secure Store. I generated an external list in my Site Collection and finally a workflow to write a record from my internal SharePoint list to the external list.

Workflow line 1: Create item in MyExternalListName (Output to Variable: create).

I am able to select all columns in value assignment except for the primary key. Is this a restriction?

Thank you!