JuxLauncher
Play on PrismForge
Open JuxLauncher: it will install Minecraft 1.21 and will immediately log into the server.
The browser will ask whether to open the application - allow it.
Nothing opened?
So JuxLauncher is not worth it or is it older 1.6.12. I don't know. Now you're gonna need to update it with a yellow "update" under the logo and start again. No, put it down. It's a minute. Then press "open" again.
Free, open source, for Windows, macOS and Linux. What the launcher can do