Question How to setup DKIM for subscriptions with external mail

[wwwAndries]

A subscription, example.com, uses an external mailservice like Office 365, and there are mails sent through contact/order forms on the website.

How can I get DKIM working in this situation for mails sent through contact/order forms on the website?

I cannot enable the mailservice because mail sent to external service is delivered locally on Plesk when the mailservice is enabled.
But to make DKIM work for mails sent from the website I have to enable the mailservice.

CentOS Linux 7.8.2003
Plesk Obsidian Version 18.0.28 Update #3
Dovecot 2.3.10.1-2.centos.7
Postfix 3.4.8-2.centos.7