A bit of extra information on enabling DX12 and checking if it is properly used:
The launcher needs to be running the latest version (2057442)
DX12 is enabled by default on Singularity, if it is supported. You can disable it through the launcher settings, if you want to test something without dx12, for example if you want to compare something:
Your video card needs to support feature level 12_0, which is unfortunately not the case for the very first dx12 cards. Most video card manufacturers started to support 12_0 in 2015. You can check, if your card supports this, by opening dxdiag and checking the Feature Levels in the Display tab:
In the client you can confirm that DX12 is being used by opening the FPS Monitor through CTRL+F: