Use "in". You might want to do a bit of research to compare "in" and "at" - there are plenty of explanations online.

Thank you..!!

It dependes on the situation..but @Shawn explain it in good way.

Thank you .

In American English, I would say "I am in the seat right now." or "I am sitting in the seat right now." and "She was seated in the corner." An instance where you would use "at" is in this sentence: "He was seated AT the head of the table."