Pi-Hole is set up, how to verify that it works and adding new sites

You don't see ads when browsing with that device. You see queries from the device in the Pi-hole query log, and some of the queries are blocked.

A good website to use to test ad-blocking is cnn.com. Without an ad-blocker, you will see many ads. With Pi-hole active, you should see none.

With Pi-hole:

Without Pi-hole:

What kind of entry do you want to make? Are you trying to add an adlist (public blocklist), or a local domain block (blacklist)?