Not only with the engine running and in park, but also the car needs to be warmed up when you check it. There is much expansion of the fluid.
You add it into the dipstick hole. You can get a special long funnel for the purpose.
I have two items of caution about this:
1. DO NOT USE A FUNNEL THAT HAS BEEN USED TO PUT IN ETHYLENE GLYCOL ANTIFREEZE. The antifreeze is absolute poison to the AT clutches. A plastic funnel will hold enough traces of ATF to ruin clutches regardless of how well you wipe it off. Get a funnel that works with your AT and mark it "ATF ONLY."
2. DO NOT OVERFILL!!! You can easily blow out the seals when overfilling an automatic transmission.