How to calculate calories to lose weight

No, that would be dangerous.

If you put your descriptive data into your MFP profile, and tell it you want to lose X amount of weight per week, it gives you a calorie goal. Eat that many calories.

MFP wants you to set your "activity level" based on your daily humdrum stuff, like job and home chores, excluding intentional exercise. When you exercise, it wants you to log the exercise, and eat those calories, too. That would be expected to keep you at the weight loss rate you requested.

You burn calories all day long. For most of us, the biggest number is our basal metabolic rate (BMR), the amount we'd burn flat out in bed in a coma, not even digesting food. The next biggest part for most is daily life activity, or NEAT (non-exercise activity thermogenesis) - that job and home chores kind of stuff. Exercise is typically the third in magnitude, for a typical person.

To lose weight, you need to eat fewer calories than you burn. You burn the total of all that stuff in the previous paragraph (plus some truly tiny stuff I left out). That's called your TDEE (total daily energy expenditure). When it estimated your calorie goal, MFP has already subtracted calories from that to trigger weight loss for you.

Don't try to exercise away every calorie you eat: That would be like eating zero calories, which would be tremendously unhealthy (literal starvation), not to mention impossible to stick with for very long.

If you put a weight loss goal in your MFP profile, and it said to eat 1200, eat 1200. If you exercise, make a sensible estimate of those calories, and eat those too.

P.S. Unless you're petite, older, and quite inactive, 1200 may be too low already. Too many people set a goal of two pounds or one kilo per week loss, which is very aggressive unless well over 200 pounds (for two pounds a week) or 100 kilos (for one kilo per week). Fast loss increases health risks.

Best wishes!