Migration template : Specify custom ID Map Plugin (Drupal 8)

node_page.yml

id: node_page
label: "Page Content"
migration_tags:
  - content_import
# Notice ID Map plugin here.  
idMap:
  plugin: content_update
# ID Map plguin declaration ends.  
source:
  plugin: csv
  path: 'public://example.csv'
  header_row_count: 1
  keys:
    - 'Item-ID'
  fields:
    Item-ID: Unique identifier for each page.
    Page name: Name of the page.
    Page category: Page category.
process:
  type:
    plugin: default_value
    default_value: page
  title: Page name
  langcode:
    plugin: default_value
    default_value: "en"
  field_page_category:
    -
      plugin: skip_on_empty
      method: row
      source: Page category
      message: "Row skipped due to empty Page category"
  field_page_unique_id:
    -
      plugin: skip_on_empty
      method: row
      source: Item-ID
      message: "Row skipped due to empty Item-ID value"    
destination:
  plugin: entity:node
  translations: true
migration_dependencies:
  required: {}