On button click, redirect users to registration page instead of another page

You’ll need to find out what theme template file is being used to display the /portfolio/ page – page.php, page-portfolio.php, tpl-portfolio.php, or something along those lines.

Then, if you don’t already have a child theme, create one.

Then, copy the template file into your child theme, and at the very top – before anything else – add:

// If the user is not logged in
if( !is_user_logged_in() ) {
    // send them to the login screen
    // once they log in, they will automatically return to this page
    auth_redirect();
} else {
    // move all your other code into this condition
}

As the comments note, if the user isn’t logged in, they’ll be redirected to the login page. Once they are logged in, they’ll be redirected back to the portfolio page.