Exercise

It depends. There is a HIIT entry in the exercise database, but I'm skeptical about it.

Here's the root of the problem: HIIT isn't exactly a type of exercise, it's an exercise pacing strategy. "HIIT" stands for "High Intensity Interval Training". We can do HIIT circuit training with weights or weight machines, HIIT cycling, HIIT running on a treadmill or outdoors, HIIT calisthenics, and more. These days, there are "HIIT classes" but they can be any of a variety of those or other things.

What we really want to do in MFP is log the exercise as the type of exercise it is. That's more closely correlated with how many calories it burns.

So: What do you do in Planet Fitness's HIIT Workout?

If it's more like weight training done at high reps with relatively lower weights/resistance, use one of the "Circuit training" options in MFP.

If it's more like calisthenics, with lots of bodyweight-type exercises, use one of the "Calisthenics" entries.

If it's more of a dance-y vigorous thing, consider the "Aerobics" entries.

And so forth. Figure out which exercise type is closest to what you did, and log it as that. I'd suggest that if there are multiple options of that type to pick between, pick one of the ones that adds relatively lower calories, because it's more common to over-estimate exercise calories than under-estimate them. If you seem to be losing weight too fast on average after a few weeks, eat more calories, whether you do that by using a higher-calories exercise entry, or just by eating more calories.

Someone is probably going to come along and tell you to get a heart rate monitor or fitness tracker, and use its estimate. HIIT - really any kind of interval training or strength training, but especially HIIT - is not really well-estimated by using heart rate to estimate. I can explain why that might be so, but I'm not going to belabor that here unless asked. If someone has a good brand/model fitness tracker and syncs that to MFP, that may be OK, even with some HIIT in the picture . . . but just using the HRM/tracker's exercise estimate to log manually has some possible downsides.

TL; DR: Figure out the closest exercise type, log it as that. Don't log it as something that's really a pacing strategy.

Do you have a fitness watch? For something like this I would turn my watch on and tell it I was doing “mixed cardio” or something generic like that then let the watch use my heart rate to estimate the burn.