MultiChain automaticallly assigns two types of unique ID to assets that are created - the asset reference and issuance txid. Either of these can be used in place of the asset name when referring to it in the APIs. While asset names generally need to be unique, you can have as many assets as you like with the empty name, so if you're just looking for some kind of unique ID, you should give the assets an empty name and rely on these auto-generated IDs.