Using an Android browser downloaded though the Play Store/sideloaded into WSA, download the following Magisk Module to install and symlink BusyBox with its applets to the Magisk built-in busybox binary. No need to do it manually or to get a Custom Build.
Through Magisk Manager, install the Magisk module .zip file
You can then test if BusyBox is working by either using a Root Checker:
or by using ADB Shell and:
adb shell
su
busybox | head -1