Prepopulate Form With Query String Parameters

This is solved by myself.

I fetched the query strings, stored them in variables and then after the iframe is loaded added the values to correspondent fields.

Cheers!

Sure.

// Fetch Query Strings
jQuery(document).ready(function($){
  function getParameterByName(name, url) {
    if (!url) url = window.location.href;
    name = name.replace(/[\[\]]/g, '\\$&');
    let regex = new RegExp('[?&]' + name + '(=([^&#]*)|&|#|$)'),
        results = regex.exec(url);
    if (!results) return null;
    if (!results[2]) return '';
    return decodeURIComponent(results[2].replace(/\+/g, ' '));
  }

//Store them in variables
  let total = getParameterByName('total');
  let email = getParameterByName('email');
  let full_name = getParameterByName('full_name');

/* 
Since I have different scenarios
and multiple forms (product shortcodes) on single page
I am showing different form based on total amount 
and want the form to open automatically
*/

if (total == 165.00) {
setTimeout(function() {
        $(".first-product-class").trigger('click');
           $(".asp-popup-iframe").load (function(){
           $('.asp-popup-iframe').contents().find('#email').attr('value', email);
          $('.asp-popup-iframe').contents().find('#billing-name').attr('value', full_name);
        });
    },10);
}

else if...

});