Microwave method: Add apple cider and light brown sugar to a heatproof bowl and microwave on high until it boils, 3-5 minutes depending on the power of your microwave. Carefully remove from the microwave and stir until the sugar is completely dissolved. Cover with plastic wrap and cool to room temperature. Transfer to a glass or plastic squeeze bottle and refrigerate.
Stovetop method: Note: You can also make this on the stovetop. Add the brown sugar and apple cider to a small saucepan set over high heat. Bring to a boil. Stir to dissolve all the ingredients. Cover and cool to room temperature. Transfer to a glass or plastic squeeze bottle and refrigerate.