Get Customer infos in webform

I want to create a webform that is linked to a customer but i dont understand how to auto fill the customer field to the one liked with the user that fill the form.
i can make an AJAX call to retrive the customer but i will have to give to much authorization to the customer role. For the Issue and Address Web Form for example, when the user fill it the customer is automaticaly assign to the new issue/doctype created but i don’t understand how it is performed.
i saw this post but i already dont understand
Thanks for your help and for this Amazin Framework !