That looks like a top fill Lemo
You twist the top portion of the tank, it exposes a hole where you squeeze your juice in carefully, than twist the top back to close the hole and viola!
Welcome to V/U. In little time you'll have this down pat, just keep asking questions and you'll get plenty of help here. plenty of kind and helpful folks here!
You may find the top fill ring really tight to twist open, but that's how it's done. A RTA seems like and odd place to start for a beginner, but that is a great, RTA, I love mine.