To add a custom tracking domain on Ionos domains:
Go to your Control Panel and select the Domains section.
Click the gear icon next to your domain and navigate to Actions and select DNS.
Click Add Record and select the CNAME type.
Enter the following details.
Hostname: inst
Points to: prox.itrackly.com
Click Save.
Go to your Instantly Email Account Settings.
Click 'Use Custom Tracking Domain.'
Add the new CNAME in Instantly settings and click Check status.
If everything is set up correctly, you will receive a notification below the Check Status button: CNAME Verified SSL Verified.
Once checked, click Save.