Issue Postfix: how to set DKIM to send email from another domain on the same server

[MHC_1]

Server operating system version

almalinux 9

Plesk version and microupdate number

18.0.74

Two domains: Both on same server and same subscription.

subdomain.domain1.co.uk
And
domain1.co.uk

How to send emails from scripts on account subdomain.domain1.co.uk and make them look like they come from domain1.co.uk

The From mailbox is operational ([email protected]) . The mail server works as is otherwise.

But mail refuses to send because:

NOQUEUE: reject: RCPT from localhost.localdomain[::1]: 554 5.7.1 <[email protected]>: Relay access denied;

SPF is in place. DKIM is in place but can't see where or how to adjust DKIM to set a specific d= tag for referencing that the subdomain can send representing the main domain.