When you want to add custom tracking domain and your domain provider is Hostgator, you should follow the steps from below:
Log in to cPanel
Look for the Domains section, then click Zone Editor
On the next page, locate the domain name you need to edit
Click +CNAME Record
Create the CNAME record
Click "Add a CNAME Record"
In Instantly, under Account Settings, add the new CNAME
Add it as inst.yoursendingdomain.com in Instantly with yoursendingdomain.com being the domain you added the CNAME record under.
Click on Check Status
If everything is set up correctly, in 5-10 minutes after you created your CNAME record in your domain DNS Zone, you will receive the following notifications below the Check Status button:
CNAME Verified SSL Verified
Go ahead and save changes