Source code for https://formidable-masterminds/pass-option-in-query-string-set-select-value/

default-value-by-option-param.js

<script>
jQuery(document).ready(function($) {
    "use strict";
    
    function getParameterByName(name, url) {
        if (!url) url = window.location.href;

        name = name.replace(/[\[\]]/g, "\\$&");

        var regex = new RegExp("[?&]" + name + "(=([^&#]*)|&|#|$)"),
            results = regex.exec(url);

        if (!results) return null;

        if (!results[2]) return '';

        return decodeURIComponent(results[2].replace(/\+/g, " "));
    }
    
    $( function() {
	/* setup the variables we need to process */
	var this_url = window.location.href;
			
	/* only process pages that pass the correct query string
	* change "my_option" to the name of the parameter you're passing
	* to this function in the query string */
	if ( this_url.indexOf("my_option") ) {
			
		/* get the query string value */
		var option_name = getParameterByName( "my_option", this_url );
			
		/* find the corresponding value for this option */
		/* select field (Dropdown) */
		$("select[name='item_meta[248]'").children().each( function() {
	        	if ( $(this).text().trim() == option_name ) {
	                $(this).parent().val( $(this).val() );
	            }
	        });

	        /* radio buttons */
		$("input[name='item_meta[249]'").each( function() {
			if ( $(this).parent('label').text().trim() == option_name ) {
	                	$(this).prop("checked", true);
	           	 }
	        });

	        /* checkboxes */
		$("input[name='item_meta[250][]'").each( function() {
	        	if ( $(this).parent('label').text().trim() == option_name ) {
	                	$(this).prop("checked", true);
	            	}
	        });
	}
    });
});
</script>