Update metafield value by Ajax api Or API

Topic summary

Goal: Save a customer’s date of birth at newsletter signup and store it in a Customer metafield in Shopify.

Current state: A Customer metafield for birthday has been created via Shopify Metafield settings. Email is currently captured and saved using Shopify’s Ajax API during newsletter subscription.

Key need: How to write the birthday value to the Customer metafield via an API from the storefront at the same time as the email submission.

Context: Custom Storefronts (Liquid). Metafield = a custom field attached to a resource (here, Customer). Ajax API = Shopify’s storefront endpoints used to handle actions like newsletter signups.

Outcome: No solution provided yet; the request seeks guidance on the correct API/approach to persist the birthday into the Customer metafield. Discussion remains open.

Summarized with AI on February 8. AI used: gpt-5.

Hello ,
I want to add Birthday of Customer in Shopify.
The exact Functionality i Want is when user Subscribe From Newsletter then also Enter the Date of birth And Save in Store.
So i have Created new metafield to Customer From Shopify Metafield Settings. but confussed that how to save birthday value by api. Currently Email is store by Ajax api.

Thanks In Advanced.