curl --request PUT \
  --url https://api.lemlist.com/api/sequences/{sequenceId}/steps/{stepId}/sender \
  --header 'Authorization: Basic <encoded-value>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "userId": "usr_A1b2C3d4E5f6G7h8K"
}
'

import requests

url = "https://api.lemlist.com/api/sequences/{sequenceId}/steps/{stepId}/sender"

payload = { "userId": "usr_A1b2C3d4E5f6G7h8K" }
headers = {
    "Authorization": "Basic <encoded-value>",
    "Content-Type": "application/json"
}

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

print(response.text)

const options = {
  method: 'PUT',
  headers: {Authorization: 'Basic <encoded-value>', 'Content-Type': 'application/json'},
  body: JSON.stringify({userId: 'usr_A1b2C3d4E5f6G7h8K'})
};

fetch('https://api.lemlist.com/api/sequences/{sequenceId}/steps/{stepId}/sender', 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/sequences/{sequenceId}/steps/{stepId}/sender")

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

request = Net::HTTP::Put.new(url)
request["Authorization"] = 'Basic <encoded-value>'
request["Content-Type"] = 'application/json'
request.body = "{\n  \"userId\": \"usr_A1b2C3d4E5f6G7h8K\"\n}"

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

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://api.lemlist.com/api/sequences/{sequenceId}/steps/{stepId}/sender",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "PUT",
  CURLOPT_POSTFIELDS => json_encode([
    'userId' => 'usr_A1b2C3d4E5f6G7h8K'
  ]),
  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;
}

{
  "data": {
    "step": {
      "sequenceId": "seq_A1b2C3d4E5f6G7h8J",
      "stepId": "stp_A1b2C3d4E5f6G7h8I",
      "stepType": "email",
      "sender": {
        "userId": "usr_A1b2C3d4E5f6G7h8K",
        "channel": {
          "type": "email",
          "id": "usm_A1b2C3d4E5f6G7h8L",
          "alias": "john@example.com"
        }
      }
    },
    "senderPoolChange": {
      "enabledChannelTypes": [],
      "disabledChannelTypes": [
        "email"
      ]
    }
  }
}

{
  "error": "The channel does not match the step: give a channel of the step type, or no channel on a call, manual, API or phone step",
  "code": "SENDER_STEP_CHANNEL_MISMATCH"
}

The authentication you supplied is incorrect

route is available starting emailPro plan

{
  "error": "Step not found",
  "code": "SEQUENCE_STEP_NOT_FOUND"
}

{
  "error": "Route bad method",
  "code": "ROUTE_BAD_METHOD"
}

{
  "error": "This step type has no sender",
  "code": "SENDER_STEP_NOT_FORCEABLE"
}