For Pokedex 3D Pro, just create a quiz using the keyword PHSKUTDF, then proceed to complete the quiz. It has been reported that the results don’t matter, but I haven’t confirmed this yet.
For Pokedex on iOS, all you have to do is search for the keyword SNMPSCTP.
Have you added this mythical monster to your dex yet? Was it as simple as the instructions above make it sound? What will be released next? Tell us in the comments below!