This paper proposes a practical method to watermark behavioral IP (BIPs) for High-Level Synthesis (HLS), such that the watermark can be unequivocally retrieved at the generated RTL code, while being unremovable. The main approaches to watermark BIPs so far have focus on modifying the HLS process by e.g. introducing watermarking-aware scheduling or register binding algorithms. The main problem with these approaches is that they involve having full control over the HLS tool's internal behavior, which is not practically possible. Specifically, state-of-the-art HLS tools do not allow this type of controllability. Hence, these approaches are currently impossible to be implemented. On the other hand, commercial HLS tools make extensive use of synthesis directives in the form of pragmas. In this work we make use of these synthesis directives to assign operations in the source code to specific functional units given in the technology library in order to create the watermark. Experimental results show that our proposed method is effective in creating strong watermarks, while practical at the same time.