This feature is not directly available on Webform module. You've to implement some code to prefill the form fields.
A simple method is to write some JS which reads the URL parameters and set the values to the corresponding fields.
There are many ways to implement this code. This is an example of JavaScript code:
var current_url = window.location.href;
if (current_url.indexOf('foo=') > 0){
$('#field-id').val(current_url.substr(current_url.indexOf('foo=') + 4));
}
At first the script get the current url and if the "foo" parameter is found, then the value will be loaded and used to set the #field-id value. #field-id is the id of the input field on the webform. 4 is the length of "foo=".
If you have many parameters you can use
current_url.split("&")
method which returns an array with all the parameters.