Autofill using javascript got remove when tap on text input, how to solve this issue?

Hello Javascript programmers!

I don't know about javascript but I write a code to autofill form in this https://www.adidas.com/us/delivery site. that successfully fill the form.
But when I tap on text input all text input fields got blank automatically,

Here's that strange behaviour Gif.

But I want that fields to remain fill, when I tap on that.

Here's the javascript that I've used.

document.getElementsByName('shippingAddress.firstName')[0].value = 'Aafaq'; 
document.getElementsByName('shippingAddress.lastName')[0].value = 'Ahmed'; 
document.getElementsByName('shippingAddress.address1')[0].value = '553  Shinn Street'; 
document.getElementsByName('shippingAddress.city')[0].value = 'New York'; 
document.getElementsByName('shippingAddress.zipcode')[0].value = '10021'; 
document.getElementsByName('shippingAddress.phoneNumber')[0].value = '+1-202-555-0123'; 
document.getElementsByName('shippingAddress.emailAddress')[0].value = 'anymail@gmail.com'; 

I have to use this code with swift iOS, can anyone tell me how I can solve this issue? and what is happening under the hood?

Any help would be appreciated. Thanks in advance!.