If your full addon requires ULX be loaded (not just some small function of it), you could also place your addon file in <your_addon>/lua/ulx/modules</cl|/sv|/sh/&|>/file.lua.
Then it would like when ULX loads.
That gets tricky though if your full addon doesn't require it; but only that small command - then it becomes a 'which loads first, my code or ULX'. Luck of the draw at that point.
You wouldn't want the function to run as ULX is loading if your other code hadn't loaded first.