Marcus Tavernier had Bournemouth ahead midway through the first-half via a penalty before Kostoulas struck a superb overhead goal a minute into injury time for Brighton's equaliser.
"For you the first time you see it (his overhead kick), but Lewis (Dunk) has seen it a lot of times in training," said Kostoulas.
"I like this way to score or try to score. Today, the ball went into the net. I'm happy for this goal but the result could be better for us. We need this three points to keep pushing until the end of the season."
On how he's finding playing in the Premier League, he added: "It's an experience from a kid I want to live. I'm happy. I'm adapting day by day, and I like to play lots of positions, but mostly I like to be on the pitch everywhere and help my team."