For now the issuefrom API is designed for per-asset metadata only, which has a key-value structure (text-only, not full JSON).
So you have two good options – either pass the hex object as a field within this key-value structure, but this only works if the hex item is small (say up to 3 kB to leave space for other metadata fields within the 4 kB limit).
Otherwise, use createrawsendfrom to perform the issuance together with an additional raw metadata field in a single transaction. To see an example, you can follow the tutorial for issuance with raw transactions ( https://www.multichain.com/developers/raw-transactions/ ) and add an extra item to the data parameter containing the raw hex. You can also look at how we allow a document to be uploaded together with an asset issuance in the MultiChain Web Demo: https://github.com/MultiChain/multichain-web-demo/blob/master/page-issue.php