Populating checkout fields

Topic summary

A developer has created a custom city dropdown on the Shopify checkout page using UI checkout extensibility. They need to synchronize this custom dropdown with Shopify’s native city field so that when a user selects a city from the custom dropdown, it automatically populates the standard Shopify city field.

The question remains unanswered, with no solutions or workarounds provided yet. This appears to be a technical integration challenge involving Shopify’s checkout extensibility API and native form field manipulation.

Summarized with AI on October 30. AI used: claude-sonnet-4-5-20250929.

I added my own city dropdown to the checkout page using the UI checkout extensibility, any idea how I can make the changes to this dropdown reflect in the native Shopify city field? That is populating the field itself upon city selection in the dropdown created by the app.