‘Moderately active’ may not mean what you think it does. Farmers and professional athletes are ’moderately active’. People who work at home all day using a computer are ‘sedentary’.

(Given your gender, age, height and weight:) Unless you work out for >4 hours every day, 3500 kcalories is way more than your body uses, so you’re sure to gain weight if you were to ingest that many calories every day. If you have a desk job or if you’re a student, choose ‘sedentary’ in the app. The result will be 2300 kcalories, which is most likely enough for you. On days in which you workout a lot, you can ingest a few hundred kcalories more.

To add to what Samuel_Michon wrote, I'm 38, 103kg at the moment (I'm substantially above averagely muscular, but also somewhat overweight now), 1m 85, and I exercise hard 5 days a week - 1 hour of heavy weights 3-4 times/week, and cardio the rest. I work in an office, so my work day is fairly sedentary. I also have a young son which keeps me more active than I've ever been before during the evenings.

I burn ~2800 kcal/day when I exercise.

This is an estimate based on tracking every meal, and tracking my weight every morning, rather than what I "should" be burning.

Unless you have a very active job, or exercise hard (think stumbling out of the gym) for substantially more than an hour a day, or spend every moment of your spare time being highly active, you're likely not burning 3400 calories.

But really, write down what you eat for a couple of weeks, count the calories, track your weight, then adjust the number of calories to target up/down depending on whether you want to increase/decrease your weight, as there are so many factors that can change what how much you need.

Also, contrary to what you might think, even spending an hour per day in the gym weight lifting is not burning a lot of calories. It won't take you from 2300 calories expended per day to 3400, for instance.

One hour of weight training for a 200 lb man is only 450 calories burned, or a Medium Fries at McDonalds. You have to really be running or other high cardio sports to burn even 1000 calories per hour.

Working out has also been shown to cause people to eat more subconsciously, so those 450 calories burned end up being only 100-200 net calories burned once you've had the protein shake after the hard workout.