area: email
action: mergelist
https://api.hypermail.com/api.php?apikey=YOURAPIKEYHERE&area=email&action=mergelist&listids=2,4,6&title=newlisttitle
<response>
<success>List Merge in progress. Use a notification webhook to get the ID of the newly created list</success>
</response>
{
"success": "List Merge in progress. Use a notification webhook to get the ID of the newly created list"
}