I just tried this today with firmware 4.0.2 (lost root after firmware upgrade), and had to use edl to write the patched image to the device - neither 'fastboot boot' nor 'fastboot flash' worked, both failed with 'unknown command'.
The command I used on linux:
Code:
edl w boot_b magisk_patched.img --loader=Qualcomm_662_loader.elf --debugmode