curl --request POST \
  --url https://api.lemlist.com/api/contacts/crm/create \
  --header 'Authorization: Basic <encoded-value>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "idsOrEmails": [
    "ctc_xW8Ou6C03Csv8vatp",
    "john.doe@acme.com"
  ],
  "listId": "clt_Ab12Cd34Ef56Gh78I",
  "asLead": false
}
'

import requests

url = "https://api.lemlist.com/api/contacts/crm/create"

payload = {
    "idsOrEmails": ["ctc_xW8Ou6C03Csv8vatp", "john.doe@acme.com"],
    "listId": "clt_Ab12Cd34Ef56Gh78I",
    "asLead": False
}
headers = {
    "Authorization": "Basic <encoded-value>",
    "Content-Type": "application/json"
}

response = requests.post(url, json=payload, headers=headers)

print(response.text)

const options = {
  method: 'POST',
  headers: {Authorization: 'Basic <encoded-value>', 'Content-Type': 'application/json'},
  body: JSON.stringify({
    idsOrEmails: ['ctc_xW8Ou6C03Csv8vatp', 'john.doe@acme.com'],
    listId: 'clt_Ab12Cd34Ef56Gh78I',
    asLead: false
  })
};

fetch('https://api.lemlist.com/api/contacts/crm/create', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

require 'uri'
require 'net/http'

url = URI("https://api.lemlist.com/api/contacts/crm/create")

http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true

request = Net::HTTP::Post.new(url)
request["Authorization"] = 'Basic <encoded-value>'
request["Content-Type"] = 'application/json'
request.body = "{\n  \"idsOrEmails\": [\n    \"ctc_xW8Ou6C03Csv8vatp\",\n    \"john.doe@acme.com\"\n  ],\n  \"listId\": \"clt_Ab12Cd34Ef56Gh78I\",\n  \"asLead\": false\n}"

response = http.request(request)
puts response.read_body

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://api.lemlist.com/api/contacts/crm/create",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
  CURLOPT_POSTFIELDS => json_encode([
    'idsOrEmails' => [
        'ctc_xW8Ou6C03Csv8vatp',
        'john.doe@acme.com'
    ],
    'listId' => 'clt_Ab12Cd34Ef56Gh78I',
    'asLead' => false
  ]),
  CURLOPT_HTTPHEADER => [
    "Authorization: Basic <encoded-value>",
    "Content-Type: application/json"
  ],
]);

$response = curl_exec($curl);
$err = curl_error($curl);

curl_close($curl);

if ($err) {
  echo "cURL Error #:" . $err;
} else {
  echo $response;
}

{
  "success": true,
  "data": {
    "alreadyLinked": 2,
    "toCreate": 6,
    "notSearchable": 1,
    "alreadyInProgress": 0,
    "notFound": [
      "jane.doe@acme.com"
    ]
  }
}

{
  "success": false,
  "error": {
    "code": "NO_CRM_CONNECTED",
    "message": "No CRM connected for this team"
  }
}

The authentication you supplied is incorrect

{
  "success": false,
  "error": {
    "code": "CREATE_IN_CRM_NOT_ALLOWED",
    "message": "Only a team admin can create records in the CRM"
  }
}

{
  "success": false,
  "error": {
    "code": "LIST_NOT_FOUND",
    "message": "List not found"
  }
}