update metafield value by Ajax api Or API.

[axy]

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.