P90x

You're can't unless you wear a HR monitor or some other device. MFP doesn't do a good job of even guestimating this stuff IMO, so I use the UA Record app and sync it with MFP. It has a category for Workout Videos, just inout how length of your workout and it gives you an estimated calorie burn based on your stats (weight, height, etc)

Here you go! (link at the bottom)

Scroll all the way to the bottom of the page, enter your weight and other details, select which workout from the P90X program you did and there you go! Calculates how many calories you've burned. And then you can add it as a custom exercise on here to log it

https://p90xcalories.com/