Simply adding an Environment Map automatically enables a skydome_light with no other input required. The 'skydome_light' and background shader is constructed under the hood, and no extra light creation is required.
The lights page has more detail about the controls. The only additional parameters are:
The skydome_light has a texturable color slot so that you can map a texture to be used for IBL.
The resolution attribute should be used with care. The higher the resolution value, the longer the skydome_light will take to precompute the importance tables for that light. For high-resolution maps, this can be very slow.