Using Event Tickets, we have an option for requiring users to log into their accounts for registering for an event. When this option is enabled, a button for notifying users will be displayed in the ticket/RSVP block.

By default, this button will take users to the default WordPress login page.

Often, you might find this unsuitable, and you may rather send users to a custom login page; the good news is that it would be possible with a bit of customization.

As the first step, you need to install and activate a plugin for creating a custom login/registration page. You can find the list of recommended plugins here.

Most plugins provide shortcode for creating a custom login page, or at least have a predefined URL/page for it. 

Once you know the custom login page URL, it’s time to add a snippet to do the trick:

All you need to do is replace /new_login_page with the actual URL. After logging in, users will be redirected back to the event page where they started.

See this article for using custom code snippets.