Retrieve querystring values from the uri, strip out encoded characters and add them to form fields with equivalent names. I'm using this to pre-populate a a generic contact form with dates and details based on which link is clicked.

autopopulate.js

jQuery(function ($) {
        //grab the entire query string
	var query = document.location.search.replace('?', '');
		    
	//extract each field/value pair
	query = query.split('&');
		
	//run through each pair
	for (var i = 0; i < query.length; i++) {
		    
	//split up the field/value pair into an array
	var field = query[i].split("=");

	//decode special characters
	var cleanField = decodeURIComponent(field[1]);
	
	//target the field and assign the cleaned up value
	$("input[name='" + field[0] + "'], select[name='" + field[0] + "']").val(cleanField);
	
	}
});