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In this example we take the domainname yourdomain.com
the name of your subdomain: subdomain.yourdomain.com
1 : Login your domain registrar panel ( Namecheap, Godaddy or whatever )
2: Go to your DNS records of that domain
3: Create in your DNS records a new A record :
name: subdomain A record and point to the IP of your server
4: Be patient, the new created subdomain takes some time before it works worldwide.
5 : Login your Aapanel
6: In the menu ( on the left ) go to : Website
7 : Click the green button: Add site
8: In the field 'Domain name' fill in : subdomain.yourdomain.com and fill the other fields to your needs.
In this example change yourdomain.com and the name subdomain of course to your actual domainname!
Hope this helps.