How to link entries into a chain: add a "chain_tag" field to any of your JSON files, with any text you like — e.g. "chain_tag": "CHAIN1". You make this tag up yourself, it's just a temporary label for this one upload; you never need to know or set any real chain ID. Every file sharing the same chain_tag gets linked together automatically, in order, when you hit Import — the actual chain link is created for you. Files with no chain_tag field just import as normal, standalone entries. Use a different tag per chain (e.g. CHAIN1, CHAIN2) if you're uploading more than one chain at once.