Log in
SKlauncher has two ways to sign in. Pick the one that matches your situation.
Microsoft login
Use this if you own Minecraft: Java Edition and have signed in to it at least once via the official launcher.
Offline login
Use this if you don't own the game, or want to play with any username.
You can still join multiplayer, but only on servers with online-mode=false.
TIP
Buying the game has many advantages and supports the creators. Keep saving and grab it when you can!