curl --request POST \
  --url https://api.lemlist.com/api/inbox/linkedin \
  --header 'Authorization: Basic <encoded-value>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "sendUserId": "usr_FU6yAd3UdBFel0UDy",
  "leadId": "lea_wadZOcDDSFWSbFaCf",
  "contactId": "ctc_ei6zVayP9B2ugwUPN",
  "message": "Hi, I wanted to follow up..."
}
'

{
  "ok": true
}