When you want to thank someone for doing something(in the present moment), You should say “Appreciate it”.
Both “Appreciate it” and” “Appreciated it” are correct but it depends on the context. Most likely, you are looking for “appreciate it” as you have the feeling of gratitude now.
We use “Appreciate it” when we want to be grateful for somebody doing something for us. Let’s look at an example.
John: I wrote the Patterson report and sent it to Bill as I knew you were ill.
Peter: Oh Thanks John, I really appreciate it. I still am not right.
As you can see people usually say “I appreciate it” for something someone else did for them but nowadays people sometimes just say “appreciate it” for short.
“Got you a drink”
“Appreciated it” is also possible in English but it is less common. We use”appreciated it” when you were grateful for something in the past.
“That was a tough day, you needed a lot of help walking”.
“You’re right, I really appreciated it at the end of the day”.
We don’t often use “appreciated it” because generally, we want to show that we still appreciate something in the present. We only use “appreciated” when we want to emphasize that we were grateful for a time in the past.