What's the difference between doing this and just flashing the magisk zip?Install Beta 3, than flash TWRP, delete data, reboot into TWRP and write in the terminal from TWRP
dd if=/dev/block/platform/11120000.ufs/by-name/BOOT of=/sdcard/boot.img
After this you have the boot img on your internal storage. Install magisk apk and patch the img.
Now you can flash the patched boot.img through TWRP as img in boot section. Voila!