Yaremchuk reacted quickest after Ajax goalkeeper Remko Pasveer saved a fierce shot from Gonçalo Ramos and the Ukraine international received a yellow card for taking his shirt off during his goal celebrations.
Yaremchuk was wearing the Ukrainian coat of arms under his Benfica kit and, amid the ongoing conflict with Russia on the Ukrainian border, took to Instagram after the match to explain his celebration.
"I am Ukrainian and I am proud of that," he wrote. "Being thousands of kilometres away from my native country, I want to support everyone who is now in tension in their native land, now is the time to unite.
"This is our country, our history, our culture, our people and our borders. I would like to thank our defenders for their courage. Glory to Ukraine."
Earlier, Sebastien Haller, who has scored in every Champions League game he has played this season, put through his own goal and then found the next at the right end in a five-minute spell.
Dusan Tadic scored the opening goal of the game when he fired Ajax in front within the first 20 minutes. After playing a one-two with Noussair Mazraoui, the Serbia international struck the ball into the net from 12 yards out.
Benfica equalised on 25 minutes through Haller's misjudged clearance, but the Ivorian striker put right his previous mistake moments later when, having beaten Jan Vertonghen and drawn a from Odisseas Vlachodimos, the 27-year-old tapped in the rebound from eight yards.
With 18 minutes left, Ajax had a penalty appeal turned down, whereupon Benfica had the opportunity to counter-attack, leading to Yaremchuk's goal.
The second leg of the tie takes place in Amsterdam on March 15.