1. Changing the page URL
If the page you are talking about (ex: ABC) is on your WordPress site, it will automatically be listed in a dropdown called Pages in the Menu, like so:
From there you can check the page you want and click “Add to Menu“.
When you add a “Custom Link” to the menu, this is mainly used if you are linking external websites that is not on your WordPress website.
To change the URL slug of your page (/abc to /xyz), under the Pages section in your Dashboard, open up the page in question (ABC in this case) to edit it. When you have the page opened in your editor, at the top, you should see the following:
Where you see http://abc.wpengine.com/abc/ click “Edit” next to it. This will allow you to set the new URL slug to xyz.
2. MailChimp
Looking at your screenshot, I’m guessing that you are already implementing a newsletter plugin?
To have your subscriber’s information added to your MailChimp mailing lists, I’d look into using the following plugin: MailChimp for WordPress
There’s more details on how to use this plugin on the link that I gave you.