I would like to create a reference axis in a new (blank) assembly, before placing parts. The idea is to use the axis for placing the parts.  However, the tool for creating reference geometry in the assembly is grayed out.  I have to bring in a part first before this becomes active.  I can create the reference axis after doing that, but I was curious if there was a way to do it before.  Is there?  Seems like a reasonable thing to want to do in general, so I expect there's something I'm missing.

Thanks for any help with this.