How to join
Snapshots are test versions of Minecraft, so they're off by default in the launcher. Turn them on once and the launcher keeps you on the newest one.
Checking the server's version…
Open the Minecraft Launcher and go to Installations
Pick Minecraft: Java Edition on the left, then the Installations tab at the top.
Tick "Snapshots"
Next to the search box, turn on the Snapshots checkbox. A Latest snapshot installation appears in the list.
Play "Latest snapshot"
Go back to Play, choose Latest snapshot from the menu next to the Play button, and start the game. It updates itself whenever Mojang puts out a new snapshot.
Add the server
In Multiplayer, click Add Server and enter the address (opening soon).
Good to know
Your version has to match
If Minecraft says the server is on a different version, restart the launcher so it picks up the newest snapshot. The server may take a few days to move to a brand new one.
No Bedrock or consoles
Snapshots only exist for Java Edition, so phones, consoles and Windows Bedrock can't join.
Most mods won't work
Mods and clients like OptiFine usually aren't made for snapshots. Play the plain game from the official launcher.