area: email
action: addcustomdomain
https://api.hypermail.com/api.php?apikey=YOURAPIKEYHERE&area=email&action=addcustomdomain&domain=test.com&domainid=111
<response>
<customfromdomain>
<id>7</id>
<domain>test.com</domain>
</customfromdomain>
</response>
{
"customfromdomain": {
"id": "7",
"domain": "test.com"
}
}