Normally in UEFI (I don’t think there is even an option to boot in BIOS mode).
But how can I choose to boot in legacy bios from manjaro live USB?
And isn’t there already a UEFI version of memtest?
Starting from MemTest86 v5, the code was re-written to support self-booting from the newer UEFI platform. The software still remains free to use without restrictions. The MemTest86 v4 project is still maintained and remains open source, for use on old machines with BIOS. However, from V5 with the latest release being V8.3 the software is being released under a proprietary license. Development on the MemTest86+ version seems to be discontinued as there have been no updates since 2013
You could probably do something similar to that [HowTo] Booting manjaro iso directly with grub but this is not simple as copy pasting the original tutorial. This is just to show you a possibility.