How do I add a structured reference (OGM) correctly?

Use the numeric format, without "+++" or "/".

The structured reference (OGM) is not strictly specified in UBL, but the common practice is to provide only the numeric part, e.g., "reference": "123456789101". The notation +++123/4567/89101+++ may appear visually on a PDF attachment, but in the UBL only the numeric part is used.