Desk job

walk around on you break. trips to the rest room, photocopier etc. park further away from the office or at the far end of carpark etc. if your on public transport, get off one bus stop before or walk to the next bus stop on the way home
ultimately it's calories in calories out. input you stats into MFP. it'll give you your basic calorie allowance at a sedentary rate.
good luck

Be active outside of work.

When I went from bartending to a desk job that happened to me as well, and I had to a) add in actual exercise to my routine in the morning before work, instead of assuming that all the lifting and running around was sufficient; b) track my calories more closely since I was burning so much less and had a much smaller buffer; and c) fit some movement in where I could, either going to the gym or for a run on my lunch break, running or cycle commuting, or just taking a walk at lunch if that's all I can manage.

I currently bike to/from work (it's not far, only like 5K, so I'm doing it even through our miserable prairie Canadian winter this year - means I get some exercise without adding time to my day since it takes me just as long to get to work if I use transit or drive), run 5-10K 4 times a week before or after work on the treadmill (or outside if it's warmer than -15C or so), and fit walks in where I can (lunch if I can, after work with the dog.) I'm not into strength training so I just kind of faff about a couple times a week with my husband's basement equipment and mostly stick to the cardio I like. And even though I've been maintaining for years, I still track my intake so I don't get taken by surprise.

I'm pretty light at about 118 lbs and I still manage a TDEE of 2200-2600 most days, compared to the no-excercise 1600 or so I get, so for me the extra time and effort is worth it.

I work at a sort of call center, I caption the phone calls, I don't actually answer them. Here they track the percentage of the time that we are logged into the call program compared to the amount of time we are clocked in. If we are off calls for more than 9% of the time that we are clocked in, we will be talked to about getting that number down... So frequent bathroom breaks, walking around, etc aren't an option here. I've actually had to drink less water since I started here I used to work a much more active job and once I switched here I think I gain 10lbs in 2 weeks and then just steadily up from there. I've now lost 20 that I've gained just by tracking calories and doing what I can for movement.
They do have a few standing stations but other than that I am stuck to walking back and forth in the 3 feet radius my headphone cord allows me to travel.
I also do small movements like calf raises, flexing/unflexing muscles, and just constantly fidgeting.

I work a desk job.

What works for me:

Adopt a hobby that involves activity. I run just about daily and my husband and I often incorporate walking into our weekend date.

Walk on a lunch break. I get an extra 15-20 minutes of walking in each work day just from doing this.

If you have one-on-one meetings and your area will allow it, make some meetings walking meetings. I get an additional 1-4 hours a week of walking from doing this.

Consider a standing desk. I don't think this actually burns many calories, but it helps me avoid feeling "blah" from sitting all day.

Incorporate walking into some or all of your commute. I walk to and from work several times a week. It's 2 miles each way and those miles add up.

Following this as have similar issues. List 22 lbs with a calorie deficit and trying to get more walking and flexibility in.