That's a new one.
Welcome to the forum. My initial thought is that you're probably better off getting a dedicated karaoke machine, but what you're trying to do is possible, albeit a little tricky. Getting the receiver to output two sources from the same input (so as to allow both to be heard at the same time) would require the use of some kind of a two channel mixer upstream of the input connection on the back of the receiver. These can usually be found fairly cheap for no more capability than what you're looking for. You can then plug both the microphone and your laptop's audio input into separate channels of the mixer, and run the output into the receiver. This will also allow you to adjust the levels of each independently of each other - but not sure what the end result will be like, as I've never done anything like that through an ordinary receiver.
Note: You will probably want to look for something that has the ability to amplify your mic's signal to line level, which is needed for the standard audio inputs on most receivers, or one that has a standard microphone input.
Check this out:
Gemini Mixer
Of course there are countless ones out there other than what can be found on ebay too, best to shop around and see what might work for you.
Hope that helps!