If you need to acquire a boot.img then follow 1st
1st
Extracting a boot.img using Fastboot Enhance
Pre-requisites
A device with a working version of magisk manager installed, simply change the channel to the version outlined in the list above relevant to your firmware then select install / update:
A Windows 10 computer / laptop
A full update downloaded firmware zip for your model (recommend oxygen updater)
Payload dumper program for Windows.
The local System Update Installer : normal local system update will not allow a reboot during installation, the one I've linked will.
Process
On your computer
1. Download and copy the fastboot enhance folder to your desktop or a location of your choosing.
Download from here:
https://github.com/libxzr/FastbootEnhance/releases
2. Download the firmware of your choice and open it using Winrar or a similar zip program, extract the file called payload.bin a folder of your choosing.
3. Run the program FastbootEnhance.exe, a window will appear, click Payload Dumper, then click browse and point to the payload.bin file, the contents will now be shown.
4. One of the files will be called boot.img, this is the file we'll be patching, click extract then copy this to your phone, root of storage which is called /sdcard.
On your phone
5. Open up Magisk and click Install / Update, if you don't see update then change channel, so this by changing the channel in magisk settings, it'll then show the install/ update buttons.
6. Select "Select and Patch a file" it'll open up a file browser, navigate to the boot.img you copied earlier and select it, allow it to complete (this will output a file called "magisk_patched" (which will be in the Downloads folder)
You now have a patched boot.img ready to flash to either slot with fastboot or boot via fastboot, would recommend booting as this is risk free, if it's a bad image for whatever reason and you choose to flash then your phone may bootloop.
NOTE** Some people have noticed that an already patched boot.img when flashed can fail and cause a boot loop when flashed,so if you have downloaded a magisk_patched img from somewhere else then you WILL need to re-patch it on your own device preferably using the afore mentioned magisk manager based on what firmware it's from.