Some manufacturers sell computers that doesn’t fully comply with the UEFI specs and it’s quite difficult to install anything else than Windows there.
But before jumping into conclusions, have you checked that your system is prepared? For example, you can read this to get some points: [root tip] Dual boot Manjaro and Windows. Specially you have to be sure that Secure Boot is disabled.
In this case anyway, I would say that if you are unable to add a Manjaro entry (probably due to motherboard restrictions), and you have to rely on the device boot, then you have to modify the default boot order in your UEFI/Bios and make it the default. I can not give instructions, as UEFI/BIOS can be quite different between different computers.