How to send an automatic reply using webform? I'd like to send an automatic reply to every user who fills the form 
"Thank you for registration"+ link to a restricted webpage on my website -available only to registered users who filled that form.

Is it doable?
Yes you can do that in the following way:


Add an email field in webform. Set that field to get the users email by clicking "User email as default" option in the field's edit page.
Configure your forms to send emails by going at node/%nid/webform/emails and choosing "Component value", then choose your component.
After you add this "rule" you can then configure the actual email send to the user.


PS if you are not sure what the capabilities of a module are you can try it out at https://simplytest.me/.