How to migrate a value into a field attached to a taxonomy_term (from CSV using YML)

I’m trying to migrate a CSV file of taxonomies (cities) into Drupal 8. The CSV has a code and the city name, for example:

    City_Code,"Name"
    1234,"CITY A"
    5678,"CITY B"

I have the following as my current migration, which is working, and importing the city names. My problem is I also added a field with the machine name `field_legacy_id` to my term (Drupal 8), which I would like to populate with the OLD `City_Code` for reference later.

    langcode: en
    status: true
    dependencies: {  }
    id: cities
    class: null
    field_plugin_method: null
    cck_plugin_method: null
    migration_tags: null
    migration_group: null
    label: 'My Import'
    source:
      plugin: csv
      path: /path/to/data.csv
      delimiter: ','
      enclosure: '"'
      header_row_count: 1
      keys:
        - City_Code
      column_names:
        0:
          City_Cd: City_Code
        1:
          Name: Name
    process:
      vid:
        plugin: default_value
        default_value: city
      name: Name
      field_leagcy_id: City_Code
    destination:
      plugin: 'entity:taxonomy_term'
    migration_dependencies: null

The field_legacy_id seems to be ignored and I can’t figure out how to get the migration to populate the field value. Thanks.

This is the field attached to the `taxonomy.vocabulary.city`:

    langcode: en
    status: true
    dependencies:
      config:
        - field.storage.taxonomy_term.field_legacy_id
        - taxonomy.vocabulary.city
    id: taxonomy_term.city.field_legacy_id
    field_name: field_legacy_id
    entity_type: taxonomy_term
    bundle: city
    label: legacy_id
    description: ''
    required: false
    translatable: false
    default_value: {  }
    default_value_callback: ''
    settings:
      min: null
      max: null
      prefix: ''
      suffix: ''
    field_type: integer

(Edited for formatting)

Drupal version: 


Source: https://www.drupal.org/taxonomy/term/37/feed