Populate GiveWP fields via query string

functions.php

// Hooking into the single form view.
add_action( 'give_post_form_output', 'give_populate_amount_name_email' );
function give_populate_amount_name_email() {
	?>

	<script>
		
		// use an enclosure so we don't pollute the global space
		(function(window, document, $, undefined){

			'use strict';

			var giveCustom = {};

			giveCustom.init = function() {

				// Get the amount from the URL
				var getamount = giveCustom.getQueryVariable("amount");
				var amount = '1.00';
				// Set fallback in case URL variable isn't set
				if ( getamount !== false ) {
					amount = getamount;
				}
				var firstname = ( giveCustom.getQueryVariable("first") !== false ) ? decodeURI(giveCustom.getQueryVariable("first")) : '';
				var lastname = ( giveCustom.getQueryVariable("last") !== false ) ? decodeURI(giveCustom.getQueryVariable("last")) : '';
				var email = ( giveCustom.getQueryVariable("email") !== false ) ? decodeURI(giveCustom.getQueryVariable("email")) : '';
				var company = ( giveCustom.getQueryVariable("company") !== false ) ? decodeURI(giveCustom.getQueryVariable("company")) : '';
				var currency = ( giveCustom.getQueryVariable("currency") !== false ) ? decodeURI(giveCustom.getQueryVariable("currency")) : '';
				// Populate the amount field, then update the total
				if ( $('#give-amount').length > 0 ) {
					$('#give-amount')
						.val(amount)
						.focus()
						.trigger('blur');
				}
				if ( firstname !== false && $('#give-first-name-wrap input.give-input').length > 0 ) {
					$('#give-first-name-wrap input.give-input')
						.val(firstname);
				}
				if ( lastname !== false && $('#give-last-name-wrap input.give-input').length > 0 ) {
					$('#give-last-name-wrap input.give-input')
						.val(lastname);
				}
				if ( email !== false && $('#give-email-wrap input.give-input').length > 0 ) {
					$('#give-email-wrap input.give-input')
						.val(email);
				}
				if ( company !== false && $('#give-company-wrap input.give-input').length > 0 ) {
					$('#give-company-wrap input.give-input')
						.val(company);
				}
				if ( currency !== false && $('.give-total-wrap select.give-cs-select-currency').length > 0 ) {
					$('.give-total-wrap select.give-cs-select-currency')
						.val(currency);
				}

			}

			giveCustom.getQueryVariable = function( variable ) {

				var query = window.location.search.substring(1);
				var vars = query.split("&");
				for (var i=0;i<vars.length;i++) {
					var pair = vars[i].split("=");
					if(pair[0] == variable){return pair[1];}
				}
				return(false);

			}

			giveCustom.init();

		})(window, document, jQuery);

	</script>

	<?php
}