Redirect to requesting page after login?

Only certain posts on our site require a login. I’m trying to figure out how to redirect the user back to the post they were trying to view after logging in.

Looking at saml_client.php, I see this:

if( array_key_exists('redirect_to', $_GET) )
      {
        wp_redirect( $_GET['redirect_to'] );
      }
      else
      {
        wp_redirect(get_admin_url());
      }

But I don’t know where the redirect_to is coming from? How do I set that to whatever page they were on when they logged in?

https://www.ads-software.com/plugins/saml-20-single-sign-on/