area: leads
action: addfolder
https://api.hypermail.com/api.php?apikey=YOURAPIKEYHERE&area=leads&action=addfolder&foldername=newfolder&parent=0
<response>
<folderid>1</folderid>
<success>Folder Added</success>
</response>
{
"folderid": "1",
"success": "Folder Added"
}