Gedron asked me to add this to my FetishNightfall player window mod as soon as I released it. I just didn't have time to get around to it until today. As soon as it's approved, it will be available
here.
When you put your cursor over the area just to the right of F1, text shows up that says "Go AFK". If you click that area it will set your AFK tag and put text just to the right of F1 that says AFK in white text with a red background. If you click the area again, it will turn off your AFK tag and remove the AFK text with the red background. It will also toggle if you set your AFK tag using any other method.