curl --request POST \
  --url https://cloud.memsource.com/web/api2/v1/projectTemplates/{projectTemplateUid}/customFields \
  --header 'Authorization: <api-key>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "customFieldInstances": [
    {
      "customField": {
        "uid": "3HYnHHLkTPJfMxBCDbPXFe"
      },
      "selectedOptions": [
        {
          "uid": "3HYnHHLkTPJfMxBCDbPXFe"
        }
      ],
      "value": "<string>"
    }
  ]
}
'

import requests

url = "https://cloud.memsource.com/web/api2/v1/projectTemplates/{projectTemplateUid}/customFields"

payload = { "customFieldInstances": [
        {
            "customField": { "uid": "3HYnHHLkTPJfMxBCDbPXFe" },
            "selectedOptions": [{ "uid": "3HYnHHLkTPJfMxBCDbPXFe" }],
            "value": "<string>"
        }
    ] }
headers = {
    "Authorization": "<api-key>",
    "Content-Type": "application/json"
}

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

print(response.text)

const options = {
  method: 'POST',
  headers: {Authorization: '<api-key>', 'Content-Type': 'application/json'},
  body: JSON.stringify({
    customFieldInstances: [
      {
        customField: {uid: '3HYnHHLkTPJfMxBCDbPXFe'},
        selectedOptions: [{uid: '3HYnHHLkTPJfMxBCDbPXFe'}],
        value: '<string>'
      }
    ]
  })
};

fetch('https://cloud.memsource.com/web/api2/v1/projectTemplates/{projectTemplateUid}/customFields', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://cloud.memsource.com/web/api2/v1/projectTemplates/{projectTemplateUid}/customFields",
  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([
    'customFieldInstances' => [
        [
                'customField' => [
                                'uid' => '3HYnHHLkTPJfMxBCDbPXFe'
                ],
                'selectedOptions' => [
                                [
                                                                'uid' => '3HYnHHLkTPJfMxBCDbPXFe'
                                ]
                ],
                'value' => '<string>'
        ]
    ]
  ]),
  CURLOPT_HTTPHEADER => [
    "Authorization: <api-key>",
    "Content-Type: application/json"
  ],
]);

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

curl_close($curl);

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

package main

import (
	"fmt"
	"strings"
	"net/http"
	"io"
)

func main() {

	url := "https://cloud.memsource.com/web/api2/v1/projectTemplates/{projectTemplateUid}/customFields"

	payload := strings.NewReader("{\n  \"customFieldInstances\": [\n    {\n      \"customField\": {\n        \"uid\": \"3HYnHHLkTPJfMxBCDbPXFe\"\n      },\n      \"selectedOptions\": [\n        {\n          \"uid\": \"3HYnHHLkTPJfMxBCDbPXFe\"\n        }\n      ],\n      \"value\": \"<string>\"\n    }\n  ]\n}")

	req, _ := http.NewRequest("POST", url, payload)

	req.Header.Add("Authorization", "<api-key>")
	req.Header.Add("Content-Type", "application/json")

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := io.ReadAll(res.Body)

	fmt.Println(string(body))

}

HttpResponse<String> response = Unirest.post("https://cloud.memsource.com/web/api2/v1/projectTemplates/{projectTemplateUid}/customFields")
  .header("Authorization", "<api-key>")
  .header("Content-Type", "application/json")
  .body("{\n  \"customFieldInstances\": [\n    {\n      \"customField\": {\n        \"uid\": \"3HYnHHLkTPJfMxBCDbPXFe\"\n      },\n      \"selectedOptions\": [\n        {\n          \"uid\": \"3HYnHHLkTPJfMxBCDbPXFe\"\n        }\n      ],\n      \"value\": \"<string>\"\n    }\n  ]\n}")
  .asString();

require 'uri'
require 'net/http'

url = URI("https://cloud.memsource.com/web/api2/v1/projectTemplates/{projectTemplateUid}/customFields")

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

request = Net::HTTP::Post.new(url)
request["Authorization"] = '<api-key>'
request["Content-Type"] = 'application/json'
request.body = "{\n  \"customFieldInstances\": [\n    {\n      \"customField\": {\n        \"uid\": \"3HYnHHLkTPJfMxBCDbPXFe\"\n      },\n      \"selectedOptions\": [\n        {\n          \"uid\": \"3HYnHHLkTPJfMxBCDbPXFe\"\n        }\n      ],\n      \"value\": \"<string>\"\n    }\n  ]\n}"

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

{
  "customFieldInstances": [
    {
      "createdAt": "2023-11-07T05:31:56Z",
      "createdBy": {
        "uid": "3HYnHHLkTPJfMxBCDbPXFe"
      },
      "customField": {
        "allowedEntities": [],
        "createdAt": "2023-11-07T05:31:56Z",
        "createdBy": {
          "email": "<string>",
          "firstName": "<string>",
          "id": "<string>",
          "lastName": "<string>",
          "uid": "<string>",
          "userName": "<string>"
        },
        "description": "<string>",
        "lastModified": "2023-11-07T05:31:56Z",
        "lastModifiedBy": {
          "email": "<string>",
          "firstName": "<string>",
          "id": "<string>",
          "lastName": "<string>",
          "uid": "<string>",
          "userName": "<string>"
        },
        "name": "<string>",
        "options": {
          "remainingCount": 123,
          "truncatedOptions": [
            {
              "deprecated": true,
              "uid": "<string>",
              "used": true,
              "value": "<string>"
            }
          ]
        },
        "required": true,
        "requiredFrom": "2023-11-07T05:31:56Z",
        "uid": "<string>"
      },
      "selectedOptions": [
        {
          "deprecated": true,
          "uid": "<string>",
          "used": true,
          "value": "<string>"
        }
      ],
      "uid": "<string>",
      "updatedAt": "2023-11-07T05:31:56Z",
      "updatedBy": {
        "uid": "3HYnHHLkTPJfMxBCDbPXFe"
      },
      "value": "<string>"
    }
  ]
}