https://understanddrupal.com/articles/adding-http-request-headers-and-authentication-remote-json-and-xml-drupal-migrations

basic.yml

source:
  plugin: url
  data_fetcher_plugin: http
  # Choose one data parser.
  data_parser_plugin: json|simple_xml

  urls:
    - https://understanddrupal.com/files/data.json

  item_selector: /data/udm_root

  # This configuration is provided by the `http` data fetcher plugin.
  # Do not disclose any sensitive information in the headers.
  headers:
    Accept-Encoding: 'gzip, deflate, br'
    Accept-Language: 'en-US,en;q=0.5'
    Custom-Key: 'understand'
    Arbitrary-Header: 'drupal'

  # This configuration is provided by the `basic` authentication plugin.
  # Credentials should never be saved in plain text nor committed to the repo.
  authentication:
    plugin: basic
    username: totally
    password: insecure

  fields:
    - name: src_unique_id
      label: 'Unique ID'
      selector: unique_id
    - name: src_title
      label: 'Title'
      selector: title
  ids:
    src_unique_id:
      type: integer
process:
  title: src_title
destination:
  plugin: 'entity:node'
  default_bundle: page

digest.yml

source:
  plugin: url
  data_fetcher_plugin: http
  # Choose one data parser.
  data_parser_plugin: json|simple_xml

  urls:
    - https://understanddrupal.com/files/data.json

  item_selector: /data/udm_root

  # This configuration is provided by the `http` data fetcher plugin.
  # Do not disclose any sensitive information in the headers.
  headers:
    Accept: 'application/json; charset=utf-8'
    Accept-Encoding: 'gzip, deflate, br'
    Accept-Language: 'en-US,en;q=0.5'
    Custom-Key: 'understand'
    Arbitrary-Header: 'drupal'

  # This configuration is provided by the `digest` authentication plugin.
  # Credentials should never be saved in plain text nor committed to the repo.
  authentication:
    plugin: digest
    username: totally
    password: insecure

  fields:
    - name: src_unique_id
      label: 'Unique ID'
      selector: unique_id
    - name: src_title
      label: 'Title'
      selector: title
  ids:
    src_unique_id:
      type: integer
process:
  title: src_title
destination:
  plugin: 'entity:node'
  default_bundle: page

error.txt

[error]  Error: Class 'Sainsburys\Guzzle\Oauth2\GrantType\ClientCredentials'
not found in Drupal\migrate_plus\Plugin\migrate_plus\authentication\OAuth2->getAuthenticationOptions()
(line 46 of /var/www/drupalvm/drupal/web/modules/contrib/migrate_plus/src/Plugin/migrate_plus/authentication/OAuth2.php)
#0 /var/www/drupalvm/drupal/web/modules/contrib/migrate_plus/src/Plugin/migrate_plus/data_fetcher/Http.php(100):
Drupal\migrate_plus\Plugin\migrate_plus\authentication\OAuth2->getAuthenticationOptions()

oauth2.yml

source:
  plugin: url
  data_fetcher_plugin: http
  # Choose one data parser.
  data_parser_plugin: json|simple_xml

  urls:
    - https://understanddrupal.com/files/data.json

  item_selector: /data/udm_root

  # This configuration is provided by the `http` data fetcher plugin.
  # Do not disclose any sensitive information in the headers.
  headers:
    Accept: 'application/json; charset=utf-8'
    Accept-Encoding: 'gzip, deflate, br'
    Accept-Language: 'en-US,en;q=0.5'
    Custom-Key: 'understand'
    Arbitrary-Header: 'drupal'

  # This configuration is provided by the `oauth2` authentication plugin.
  # Credentials should never be saved in plain text nor committed to the repo.
  authentication:
    plugin: oauth2
    grant_type: client_credentials
    base_uri: https://understanddrupal.com
    token_url: /oauth2/token
    client_id: some_client_id
    client_secret: totally_insecure_secret

  fields:
    - name: src_unique_id
      label: 'Unique ID'
      selector: unique_id
    - name: src_title
      label: 'Title'
      selector: title
  ids:
    src_unique_id:
      type: integer
process:
  title: src_title
destination:
  plugin: 'entity:node'
  default_bundle: page