Adding Sprints to Exercises

Are you asking how to log sprints for calories?

Sprints are a pacing strategy, not an exercise type. Log it as the exercise type.

If it's running sprints, log it as running; if cycling, log as cycling; if rowing, log as rowing, etc.

Same general thing is true for other pacing strategies, like intervals, high intensity intervals, low/medium/high intensity steady state, etc.: Log it based on the type of exercise it is, not the pacing strategy.

Will the estimate be accurate? Assuming you're pulling an entry from the MFP exercise database, it'll be closer or further off in part based on how good or not-so-good the METS-based estimate for the activity is. Probably close enough to be useful: Since exercise estimating is pretty approximate for many exercises, estimating with a consistent method is usually more important than being exactly exact, as a practical matter, as long as it's not crazy far off.

Would a heart rate monitor or fitness tracker estimate be more accurate? Not necessarily. It depends on the type of activity, and the type of device.

If you say what activity type you're doing, you may get suggestions from folks here about how best to estimate that type of activity.