Q: MacBook password

It depends on how the Macbook is setup - there are several possible passwords that can appear on startup.

The simplest is to try enabling automatic login…

Set your Mac to automatically log in during startup - Apple Support

This is terrible for security, especially if you are the administrator, bear in mind that if you ever lose the Mac a thief could  access all your data by simply turning on the Mac.

After re-reading your post it also may be a screensaver password…

https://support.apple.com/en-us/HT204379 See the 'Lock your Screen' section & disable that setting.

In  System Preferences - Users and Groups - Login options, here you can set Automatic login on Start up.

In  System Preferences - Security & Privacy - General, here you can check/uncheck Require password on login.