Based on the provided reference (which mentions candy melts but does not explicitly state what's added to create modeling chocolate), the crucial ingredient added to melted chocolate (or candy melts) to make modeling chocolate is likely a source of glucose, such as corn syrup or glucose syrup. However, the provided context is insufficient for a definitive answer.
To provide a comprehensive answer, more context would be necessary. Generally, modeling chocolate recipes involve melting chocolate and then incorporating a specific type of syrup.
While the reference mentions adding dye to chocolate, it doesn't state that dye is a required ingredient to transform melted chocolate into modeling chocolate. It is used for color, but not consistency.
Without further information, it's impossible to specify the exact ingredient. The primary purpose of the added ingredient is to change the consistency of the melted chocolate, making it pliable and moldable.
