The Ajax players, led by keeper Jasper Cillessen, tried to unsettle the youngster, but Forrest held his nerve to blast the spot-kick into the net.
"I actually think the delay and everything going on helped me and made me concentrate," he said. "It was just great to see it going in and then for us to go on and get all three points.
"I have always said that I would hit penalties if I was asked. They came and asked me yesterday morning and I was happy to say yes. I felt confident."
The winger reckons the result can be the catalyst for Celtic to kick on in Group H. The victory elevates them into third spot but a decent result in Amsterdam two weeks tonight would give them an opportunity to get out of the group.
"We're looking forward to Amsterdam now," he said. "It was a hard game for us last night and it will be a hard game for us over there but we have a lot of confidence about us.
"The squad is strong as we saw last night when we missed three big players. I thought Beram played really well and hopefully that will give him a big boost."
The Scotland internationalist is also hopeful that he has put his injury problems behind him as he looks to fully shake off a recent debilitating viral infection.
"I feel much better," he said. "I was glad to get 90 minutes under my belt and I am feeling much better for it."