Unlock my iPhone 8 Plus that has been disabled

You don't. Once the iPhone has been disabled, you must restore the iPhone to use it again. Follow the directions in this support document. If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

You can also watch this Apple Support video for help.

Whatever passcode you set for it.

If you no longer remember it, follow the steps here: link-> If you forgot the passcode on your iPad, or your iPad is disabled - Apple Support

You will need to restore the phone using a computer. The process will erase the device. Later you can restore your data from a recent back-up. Instructions here…

If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

It is the Passcode that you normally use to open the phone.