This function returns the cPanel account's main domain.
uapi --output=jsonpretty \ --user=username \ DomainInfo \ primary_domain
{- "apiversion": 3,
- "func": "primary_domain",
- "module": "DomainInfo",
- "result": {- "data": {- "primary_domain": "example.com"
 },
- "errors": null,
- "messages": null,
- "metadata": {- "transformed": 1
 },
- "status": 1,
- "warnings": null
 }
}