How To Configure DKIM On OVH VPS

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I have an OVH VPS on which I plan to host a website, some email addresses and some storage space for data backup.

Recently when I began installations and configurations for email, I have been trying to set up DNS records for SPF, DKIM and DMARC. SPF and DMARC have been set up successfully. But DKIM is a problem. It does not show any errors during or immediately after set up.
But whenever I use an online DKIM checker to verify its existence, I just just get a message:

> No DKIM Record found

Clearly, something has gone wrong.

With the DMARC record, I had a problem until I saw people online putting the subdomain _dmarc
on this record.

Is it possible that I need to put some subdomain, e.g. _dkim, on the configuration form so that the online checker can retrieve it from my DNS server ?

If so, what exactly is the subdomain string that I should use ?