With the MCLeaks Authenticator you can enable support for MCLeaks accounts.
Just download it, start it and click the "MCLeaks" button.

MCLeaks Authenticator

MCLeaks Authenticator v4.3.0

Updated: 27.03.2020

Download for Windows Download for macos
Authenticator - Install instructions

Here you find the steps to install and use the Authenticator:

  1. Download the Authenticator for your OS
  2. Run it by ...
    - Windows users: Just run the downloaded file
    - MacOS users: Unpack the downloaded ZIP-file and open the unpacked MCLeaksAuthenticator.app file
  3. Select the button "MCLeaks" to use MCLeaks alts.
  4. To redeem and use an MCLeaks alt just insert the ALT-Token into the Username textfield and some random letters into the Password textfield of your launcher and press login.
  5. The launcher will automatically log you in with the MCLeaks alt. Have fun :)
  6. To use your own account again select the "Mojang" button.
YouTube-Tutorial for Windows users