fastboot boot TWRP.img Not working

I'm trying to just boot the TWRP image for my OnePlus 5t to then install a custom ROM. So I typed fastboot boot "C:\Users\Elias\Downloads\twrp-3.3.1-0-dumpling.img" and it seemed to work and I got the output:

Downloading 'boot.img'                             OKAY [  0.808s]
booting                                            OKAY [ 10.119s]
Finished. Total time: 10.992s

But the device just shows the text: "Fastboot Mode" and doesn't do anything. I now read on this [forum](https://forum.xda-developers.com/nexus-4/help/solved-fastboot-boot-recovery-img-t2380675) that they somehow removed it but it would still be possible with: `fastboot -c "lge.kcal=0|0|0|x" boot customrecovery.img ` but this flag doesn't seem to exist. The output is:

C:\platform-tools\fastboot.exe: unknown option -- c

My goal is just to install a custom ROM without actually flashing anything except the ROM. So if somebody knows a different way, it would be welcome as well.