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

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