If your device is already rooted or has a custom recovery (TWRP), you can dump the existing boot.img .
Open Magisk, select "Install" > "Select and Patch a file," and choose the boot.img file. download boot img magisk
fastboot flash boot magisk_patched.img (For init_boot devices: fastboot flash init_boot magisk_patched.img ) If your device is already rooted or has