Custom web page

How do I tell the system to open the page in a different tab, rather than replacing the website? (This is for a PayPal page)

It depends on how you are launching it.

For a link in html content set the link target to be _blank
In the editor edit the link and make that change.

If you are putting a custom link on the menu my logic in the code should open a new tab for any url that does not look like same url/domain as your website.

Works as expected. Thanks