Hello Salvador,

I am really sorry for the delay of this reply as it is a weekend for us here. I don't think you can and should include javascripts on email templates. It's most likely that your email will either be rejected or marked as spam / phishing or completely get stripped out.

As for including a "registration" button, I would suggest that you use a plain anchor link instead and you can direct this link to your registration page. You can edit the theme file located in /components/com_easyblog/themes/defaltl/emails/ . These are the email's theme files.