Hey andrezos,
We'd be happy to help out. We're not quite sure what you mean by "Authentication" at startup, but there's a couple of possibilities that could be in play here.
To start, if you're being prompted for a firmware password during startup, you'll find more information on this and what to do if you forgot the firmware password here: Set a firmware password on your Mac - Apple Support
If this is indeed what you're experiencing complete the following to either turn off or remove a forgotten firmware password:
How to turn off a firmware password
1. Start up from macOS Recovery. During startup, you will be prompted to enter the firmware password.
2. When the utilities window appears, click Utilities in the menu bar, then choose Startup Security Utility or Firmware Password Utility.
3. Click Turn Off Firmware Password.
4. Enter the firmware password again when prompted.
5. Quit the utility, then choose Apple menu > Restart.
If you forgot your firmware password
If you can't remember your firmware password, schedule an in-person service appointment with an Apple Store or Apple Authorized Service Provider. Bring your Mac to the appointment, and bring your original receipt or invoice as proof of purchase.
If instead, you're referring to your Mac's login password that you've forgotten, the following steps would be best equipped to assist: If you forgot your Mac login password
We hope this information helps out.
Take care!