curl --request POST \
  --url https://api.lemlist.com/api/inbox/conversations/labels/{contactId} \
  --header 'Authorization: Basic <encoded-value>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "labelIds": [
    "lbl_3bgE97gvicZ888wz4",
    "lbl_5kfD28hvjdA777xy9"
  ],
  "appendLabels": true
}
'

{
  "ok": true
}