JSFiddle - React, Tailwind, and code Playground

HTML

<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"></head><body><form action="https://demomoney.yandex.ru/eshop.xml" method="post" onsubmit="formatReceipt(this);return false;">

	        <input required="" name="shopId" value="72491" type="hidden">
            <input required="" name="scid" value="541855" type="hidden">
            <input required="" name="sum" value="1246.47" type="hidden">
            <input required="" name="customerNumber" value="оплата кошельком, когда пользователь не авторизован" type="hidden">
            <input required="" name="paymentType" value="PC" type="hidden">
			
    <input required="" name="customerContact" value="" placeholder="Укажите телефон +7NNNxxxXXxx или электронный адрес для получения чека" size="72"><br>
    
	<input name="ym_merchant_receipt" value="{&quot;customerContact&quot;: &quot;&quot;,
        &quot;taxSystem&quot;: 1,
          &quot;items&quot;:[
            {&quot;quantity&quot;: 1.154,&quot;price&quot;: {&quot;amount&quot;: 300.23},&quot;tax&quot;: 1,&quot;text&quot;: &quot;Зеленый чай \&quot;Юн Ву\&quot;, кг.&quot;}, 
            {&quot;quantity&quot;: 3,&quot;price&quot;: {&quot;amount&quot;: 200.00},&quot;tax&quot;: 3,&quot;text&quot;: &quot;Кружка для чая, шт., скидка 10%&quot;},
			{&quot;quantity&quot;: 0.3,&quot;price&quot;: {&quot;amount&quot;: 1000.00},&quot;tax&quot;: 3,&quot;text&quot;: &quot;Предоплата 30%, настольная игра \&quot;Tea Time\&quot;&quot;},
			{&quot;quantity&quot;: 1,&quot;price&quot;: {&quot;amount&quot;: 0.00},&quot;tax&quot;: 1,&quot;text&quot;: &quot;Бесплатная доставка&quot;}
                  ]}" type="hidden">
	
    <input type="submit" value="test-fz-54-with-dynamic-customerContact">	
    </form>

    <script>

    var  validateContact = function(value) {
        var emailReg = /^([\w-\.]+@([\w-]+\.)+[\w-]{2,4})?$/,
            phoneReg = /^\+7[0-9]{10,11}/;

        isEmail = value.match(emailReg);
        isPhone = value.match(phoneReg);

       ...

JavaScript

document.forms[0].querySelector('input[name="customerNumber"]').addEventListener('input',function(){
	let obj = JSON.parse(document.forms[0].querySelector('input[name="ym_merchant_receipt"]').value);
	obj.customerContact=this.value;
	document.forms[0].querySelector('input[name="ym_merchant_receipt"]').value=JSON.stringify(obj);
});
	console.log(JSON.parse(document.forms[0].querySelector('input[name="ym_merchant_receipt"]').value).customerContact);