Instead of ‘login’ and ‘password’ use your credentials from https://app.dataforseo.com/api-access

# Instead of 'login' and 'password' use your credentials from https://app.dataforseo.com/api-access \
login="login" 
password="password" 
cred="$(printf ${login}:${password} | base64)" 
curl --location --request GET "https://api.dataforseo.com/v3/serp/yandex/locations" \
--header "Authorization: Basic ${cred}"  \
--header "Content-Type: application/json" \

<?php
// You can download this file from here https://cdn.dataforseo.com/v3/examples/php/php_RestClient.zip
require('RestClient.php');
$api_url = 'https://api.dataforseo.com/';
try {
	// Instead of 'login' and 'password' use your credentials from https://app.dataforseo.com/api-access
	$client = new RestClient($api_url, null, 'login', 'password');
} catch (RestClientException $e) {
	echo "\n";
	print "HTTP code: {$e->getHttpCode()}\n";
	print "Error code: {$e->getCode()}\n";
	print "Message: {$e->getMessage()}\n";
	print  $e->getTraceAsString();
	echo "\n";
	exit();
}
try {
	// using this method you can get a list of locations
	// GET /v3/serp/yandex/locations
	// in addition to 'yandex' you can also set other search engine
	// the full list of possible parameters is available in documentation
	$result = $client->get('/v3/serp/yandex/locations');
	print_r($result);
	// do something with result
} catch (RestClientException $e) {
	echo "\n";
	print "HTTP code: {$e->getHttpCode()}\n";
	print "Error code: {$e->getCode()}\n";
	print "Message: {$e->getMessage()}\n";
	print  $e->getTraceAsString();
	echo "\n";
}
$client = null;
?>

from random import Random
from client import RestClient
# You can download this file from here https://cdn.dataforseo.com/v3/examples/python/python_Client.zip

client = RestClient("login", "password")
# using this method you can get a list of locations
# GET /v3/serp/yandex/locations
# in addition to 'yandex' you can also set other search engine
# the full list of possible parameters is available in documentation
response = client.get("/v3/serp/yandex/locations")
# you can find the full list of the response codes here https://docs.dataforseo.com/v3/appendix/errors
if response["status_code"] == 20000:
    print(response)
    # do something with result
else:
    print("error. Code: %d Message: %s" % (response["status_code"], response["status_message"]))

const axios = require('axios');

axios({
    method: 'get',
    url: 'https://api.dataforseo.com/v3/serp/yandex/locations',
    auth: {
        username: 'login',
        password: 'password'
    },
    data: [{
        country: "us"
    }],
    headers: {
        'content-type': 'application/json'
    }
}).then(function (response) {
    var result = response['data']['tasks'][0]['result'];
    // Result data
    console.log(result);
}).catch(function (error) {
    console.log(error);
});

using Newtonsoft.Json;
using System;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;

namespace DataForSeoDemos
{
    public static partial class Demos
    {
        public static async Task serp_locations()
        {
            var httpClient = new HttpClient
            {
                BaseAddress = new Uri("https://api.dataforseo.com/"),
                // Instead of 'login' and 'password' use your credentials from https://app.dataforseo.com/api-access
                DefaultRequestHeaders = { Authorization = new AuthenticationHeaderValue("Basic", Convert.ToBase64String(Encoding.ASCII.GetBytes("login:password"))) }
            };
            // using this method you can get a list of locations
            // GET /v3/serp/yandex/locations
            // in addition to 'yandex' you can also set other search engine
            // the full list of possible parameters is available in documentation
            var response = await httpClient.GetAsync("/v3/serp/yandex/locations");
            var result = JsonConvert.DeserializeObject(await response.Content.ReadAsStringAsync());
            // you can find the full list of the response codes here https://docs.dataforseo.com/v3/appendix/errors
            if (result.status_code == 20000)
            {
                // do something with result
                Console.WriteLine(result);
            }
            else
                Console.WriteLine($"error. Code: {result.status_code} Message: {result.status_message}");
        }
    }
}

The above command returns JSON structured like this:

{
  "version": "3.20191128",
  "status_code": 20000,
  "status_message": "Ok.",
  "time": "0.4305 sec.",
  "cost": 0,
  "tasks_count": 1,
  "tasks_error": 0,
  "tasks": [
    {
      "id": "11061103-0696-0120-0000-268044305ce6",
      "status_code": 20000,
      "status_message": "Ok.",
      "time": "0.2547 sec.",
      "cost": 0,
      "result_count": 94933,
      "path": [
        "v3",
        "serp",
        "yandex",
        "locations"
      ],
      "data": {
        "api": "serp",
        "function": "locations",
        "se": "yandex"
      },
      "result": [
        {
          "location_code": 2840,
          "location_name": "United States",
          "location_code_parent": null,
          "country_iso_code": "US",
          "location_type": "Country"
        },
        {
          "location_code": 21132,
          "location_name": "Alaska,United States",
          "location_code_parent": 2840,
          "country_iso_code": "US",
          "location_type": "State"
        },
        {
          "location_code": 21133,
          "location_name": "Alabama,United States",
          "location_code_parent": 2840,
          "country_iso_code": "US",
          "location_type": "State"
        },
        {
          "location_code": 21135,
          "location_name": "Arkansas,United States",
          "location_code_parent": 2840,
          "country_iso_code": "US",
          "location_type": "State"
        }
      ]
    }
  ]
}