If you use Titan Email, your emails are automatically signed with a default key.

In case you wish to create a custom DKIM, go to the Emails section, click on Manage next to the email service in question, and select Custom DKIM:

There, you will find the details of your DKIM keys. It will look similar to this:

Finally, create a new TXT record in your domain's DNS zone using the generated values.

After making these changes, give them up to 24 hours to propagate worldwide 💡