French striker Odsonne Edouard opened the scoring after 14 minutes of the first leg of the last-32 tie but 34-year-old Senegal striker Dame N'Doye levelled for the home side seven minutes after the break.
In what was Celtic's first experience of VAR in a competitive game, Russian referee Sergei Karasev used a pitchside monitor before awarding Stale Solbakken's men a spot-kick after 78 minutes following a Ryan Christie handball.
However, Forster tipped Jens Stage's penalty onto the post and away – his fourth penalty save in seven this season.
Forster said: "It's always nice to make them. It is what we play football for.
"I am on a nice run.
"I don't really overthink it, I go with my gut feeling and sometimes you are lucky and sometimes you are not so it's nice to make the save.
"There's no way he (Christie) can get his hand out of the way – it's not deliberate.
"With VAR, if it hits the arm, it's going to be a penalty. You have to take it and move on
"It was a good point in the end but it could have been more."