- Login the webadmin https://YOURSERVER/yunohost/admin/
- Click on “Domain”
- Click on “Add a domain”
- Click on “I already have a domain”
- Write “social.YOURDOMAIN.TLD”
- Click on add to validate the creation
You need to configure your DNS server properly to redirect the subdomain to your SERVER. You can use a wildcard to redirect all subdomain to your yunohost server.