It is an elliptical sentence. The full sentence would be something like: どうか幸せでいてください or どうか幸せでありますように祈ります. The meaning is "May you be happy".

どうか Means "in what way", 幸せ means "happiness", and で can be translated as "by" pretty much the same as the Russian instrumental case. So the sentence means "what is the way to be happy" in case it is in a question, if it is in a positive sentence it will be "the way to be happy is..." I hope this helps.