How checkout pull-out data from my js modal on front end?

[limraitec]

Hello Everyone,

I'm new on Shopify development, currently I'm creating a modal where customer to select a delivery location and store info into cookies and after that If customer checkout the  info provided will pre-filled on checkout how this could be possible??

I tried search checkout code on edit code but i didn't see anything it can be done thru api/app?
For experience you can visit and try the site here : 

https://t-dev-store.myshopify.com/
password :12345
Thank you