How to enable adb in bootloader mode?

That's not possible – as in Bootloader mode, there's no real OS loaded yet (see XDA Wiki and AddictiveTips). Quoting from AddictiveTips:

In literal terms, bootloader is code that is executed before any Operating System starts to run. The concept of bootloaders is universal to virtually all Operating systems that inculcates operating systems on your PC, laptop, smartphones, and other such devices. Bootloaders basically package the instructions to boot operating system kernel […]

Every Android phone has a bootloader that instructs the operating system kernel to boot normally.

^{(emphasis mine)}

As there isn't any OS running yet in this mode (especially no Android), there's no ADB that can be run. That applies all the same to "Download mode" with Samsung devices, or HBoot with HTC (just different names for the same thing).