A normal tummy tuck would include tightening the muscles to help flatten out the belly so you would not need to do anything special or try to suck in your stomach. I do occasionally do a skin only abdominoplasty where I do not tighten the muscles and that would be a different situation. Those cases are very unique , I’m sure you’re surgeon discussed this with you.
You should follow the instructions that you were given by your surgeon. I usually advise my patients to avoid specific abdominal muscle exercises such as sit ups for 3 months after abdominoplasty with muscle repair. I do not give specific advice on holding or relaxing the abdominal muscles I would advise them to do what feels comfortable to them.I would advise my patients that Kegel exercises are fine.Best wishes