Can you explain how to do this?
Depends on your wallet software. With Bitcoin Core, then
createrawtransaction has a data field you can specify which will create an OP_RETURN output. In Electrum, you can simply write OP_RETURN followed by the hex encoding of whatever data you want in the address field.
Sorry to raise this question again, but I see that Bitcoin Developer lists this as a required field in the second parameter (which is also required). If I do not want to make an OP_RETURN output, should I just pass a {"data": ""} argument? Or will I get away with leaving the object out, as I've seen Bitcoin Developer examples
over here do?