They just set up a new process in the just-released 12.1 ... follow the following graphic to the folder location here marked "Common" ...
And in that folder, create a subfolder called "LUTs".
In that folder, make (as shown above) the two folders called "Creative" and "Technical".
In the Technical folder, put those LUTs you want to use in the Lumetri Basic tab Input LUT slot.
In the Creative folder, put those LUTs you want to use in the Creative tab of Lumetri.
Now when you launch PrPro, the LUTs you've saved to those folders before launching PrPro will show in the drop-down list in alphabetical order in both locations in Lumetri. So you might want to name your LUTs or add a prefix to them so they show up either first or in groups.