Adding that option (without quotes) to the launch options in Steam should cause the first/default entry in the launch menu to launch you in DX11 mode. At least, it does for me.
If you're unsure what mode you're in, check your client log (output_log.txt). You should see one of the following in the first couple of lines:
Forcing GfxDevice: Direct3D 11
Forcing GfxDevice: Direct3D 9
...followed later by some more detailed hardware information like so:
Direct3D:
Version: Direct3D 9.0c [aticfx32.dll 8.17.10.1452]
Renderer: AMD Radeon HD 7800 Series
Vendor: ATI
VRAM: 2027 MB (via DXGI)
Caps: Shader=30 DepthRT=1 NativeDepth=1 NativeShadow=1 DF16=1 INTZ=1 NULL=1 RESZ=1 SlowINTZ=1
If both references to Direct3D say 11 rather than 9, you're in DX11 mode.
Note that if for some reason Unity doesn't detect your system as supporting DX11, it will fall back on DX9 regardless of what you specify. (If anything, I've seen it give false positives rather than false negatives; there are a few reported cards/drivers that "support" DX11 and then crash immediately because they don't actually.)
