Question about Canon RF 35mm f/1.8 IS Macro STM

The lens can NOT be attached to a DSLR. The EF lenses are backwards compatible with the new RF mount, but the other way round is not possible. This is simply due to physics, the RF lenses are designed for a smaller distance to the sensor which only the RF mount offers. EF lenses can be connected to the EOS R with an adapter, for example, because this distance (longer with EF than with RF) can be guaranteed by means of an adapter/spacer.

The lens is compatible with the new RF bayonet of the EOS R (Canon EOS R Body Pre-Order Set (Size XL) (32.30MP))

So you need an EF-RF adapter, which (to my knowledge) should be available.