The ideal way is that you register on Microsoft list (I think),

however, you could append manually your code to the vendor.dat file:
Macintosh HD/Library/Application\ Support/FontLab/Data/vendor.dat

Or put it by Python:

from robofab.world import CurrentFont
f = CurrentFont()
f.info.openTypeOS2VendorID = "CODE"
f.update()


em.

Thanks a lot Eduardo!
All the best!

Registering it with Microsoft is a good idea. FontLab periodically updates the alias.dat from the Microsoft master list, and so do vendors of font managing applications. If you register, other apps will at some point "know" what your Vendor ID stands for.