Palace won the first-leg last night 1-0 thanks to Mateta's strike.
"I'm very happy to be the first player to score for Palace in a European cup (competition)," Mateta told Channel 5. "I saw the ball in the box and I did my job.
"I was ready if the ball came to me, to touch it and to score. I got into the right body position to the goal, I hoped to score – and yes very happy (I did).
"They played very defensive and it was hard for us and they did not give us gaps, they defended really well. We scored in the second-half, now we need to go there and win again.
"We just needed to stick to the plan that the gaffer gave us and we scored in the end. The win was the most important thing. Now we have another important game and we have to focus."
Mateta also commented on Fredrikstad's plastic pitch.
"It's not really (an issue)," he added. "I started on those pitches when I was young, so for me, it is not an issue."