The online Modus advent calendar window for Dec. 3 hid a template for a letter to send to legislators calling for a stronger stance against AfD. On the next day, the calendar suggested urging big brands to get involved, with dedicated advertising. A picture of the Coca-Cola billboard was included next to the text.

After recent elections and street protests this year showed the growing strength of the far right in Germany, the poster and advent calendar came as a call for action against intolerance.

Artists have been mobilizing against the far right with sometimes spectacular and bold actions. Last year, the Center for Political Beauty, a group of artists who focus on political protest, installed a replica of Berlin’s Holocaust Memorial outside the house of Björn Höcke, a far-right politician who had called the memorial “a monument of shame.” In 2015, the same group held a burial for a woman who had perished at sea on her way to Italy and whose surviving family members were seeking asylum in Germany.

Coca-Cola didn’t make the billboard, but it did not disavow its message, either. “Not every fake must be false,” Patrick Kammerer, director of public affairs and communications at Coca-Cola Germany, said on his own Twitter account, quoting a picture of the billboard. The company later retweeted Mr. Kammerer’s message.

The retweet and Mr. Kammerer’s apparent endorsement of the sign appear to have angered AfD members, who staged a mini-boycott of Coke, at least on social media.