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:
PS if you are not sure what the capabilities of a module are you can try it out at https://simplytest.me/.
A key part of the Webform module is sending e-mails. Webform lets you customize the name and e-mail address that is used for outgoing e-mails, but often this is not enough. You cannot use Webform to set a custom "Reply-To" e-mail header, for instance.