You can't enable it on most hardware as dithering got ripped out of DX10-class hardware long ago. Intel's one of the very few that still do it (also their dithering's very, very good)

It's probably possible to have some kind of dither post-process shader to work. It definitely wouldn't be like the old texture dither with the cute overdraw feedback artifacts though, more like PowerVR dither in this case.

While leileilol is right in that you cannot obtain 100% the same feeling as the HW dithering in old graphics cards, you can get pretty close and maybe do it "better" (quantifying better as in more accurate for the purpose of removing banding) nowadays with shaders (at expense of performance). A simple search of Dithering on ShaderToy has good examples: https://www.shadertoy.com/results?query=tag%3Ddithering. I'm also thinking you can do some run-time noise based variable pattern dithering for improved effects if you have the cycles. Or use an already existing algorithm like Floyd–Steinberg.