Unable to map Issue Key when importing CSV to update existing issues

Hello,

I am a Jira Administrator, and I am trying to update an existing custom field (Domain) on existing issues using System → External System Import → CSV.

My CSV file contains:

Issue key;Summary;Domain
During the mapping step:

I can map Summary.
I can map Domain (custom field).
However, I cannot find Issue Key in the Jira field mapping list.
I expected to map:

Issue key → Issue Key
Summary → Summary
Domain → Domain
but Issue Key is not available at all.

I am using:

Jira Administration → System → External System Import → CSV
Company-managed project
Jira Administrator permissions
My goal is to update existing issues, not create new ones.

Has anyone experienced this? Is there a specific setting, permission, or Jira version limitation that would prevent Issue Key from being available during CSV import?

Thank you.