Microwave under counter in kitchen peninsula...

I've been building cabinets for my kitchen and I just installed a peninsula. The microwave is going to be under the countertop, in the peninsula. Originally, I was thinking that I'd simply run the microwave cord thru the cabinets to an outlet behind the kitchen sink, but I now realize that that would require an extension cord. Is this a bad idea, that is, having an extension cord that is. more-or-less, permanently in place? If so, what is the correct way to do this? I'd really like to use an existing outlet to power the microwave, since I've done lots and lots of wiring already (there were many electrical problems in this old kitchen) and I don't want to mess with it any more---the walls are all closed up and I'm happy with everything (else) at this point. Thanks.