Need a passcode to Magic Mouse 2

What password? This article:

If you can't connect your Magic Keyboard, Magic Mouse, or Magic Trackpad to your Mac - Apple Support

does not suggest you need a password:

Connect the Lightning cable

Connect your wireless accessory to your Mac with the Lightning to USB Cable or USB-C to Lightning Cable, wait 1 minute, then disconnect the cable. This pairs the accessory with your Mac and charges the built-in battery. To check your accessory's charge level, click the Control Center icon in the menu bar, then click Bluetooth.

You may be sent a 4-digit validation number but no one here nor Apple knows what code was sent.

Do you know where you purchased it from?

Did you purchase via credit card that you can look at your credit card history/statements?

We are just end users here. Our only advice is to track down where you purchased it from and then ask that store to look at its records to verify it is you.

http://checkcoverage.apple.com/ will at leaset help you get a purchase date.