The increasing design and manufacturing costs are leading to globalize the semiconductor supply chain. However, a malicious attacker can resell a stolen Intellectual Property (IP) core, demanding methods to identify a relationship between a given IP and a potentially fraudulent copy. We propose a method to protect IP cores created with highlevel synthesis (HLS): our method inserts a discrete birthmark in the HLS-generated designs that uses only intrinsic characteristics of the final RTL. The core of our process leverages the side effects of HLS due to specific source-code manipulations, although the method is HLS-tool agnostic. We propose two independent validation metrics, showing that our solution introduces minimal resource and delay overheads (< 6% and < 2%, respectively) and the accuracy in detecting illegal copies is above 96%.