Step 1. Go to Settings -> Email Accounts -> click on the email address for which you want to setup DKIM
Step 2. Click on "Configure DKIM Signature"
Step 3. Click on "Generate DKIM Record" to see your DKIM DNS record settings
Step 4. Follow the instructions provided and setup the DKIM TXT record in your domain hosting site's DNS Settings.
Step 5. Once you have updated your DNS accordingly, wait for a few hours, and then come back and click "Check DKIM record & Save".