What pattern for needing a key for a command on a thing?

Struggling with a concept and not sure the best way to tackle it. I’ve got a Thing that I need to command with an action, but I also need to provide a transaction ID, which changes.

I’ve got two channels, with two items, one has the transaction id, and another that sets the command state. But I need to send this as one thing. I’ve got two ways I can call the thing, either as a single payload with both values in, or having the transaction id in the path with the action in the payload.

What I can’t figure out is, how I can include two pieces of information. I’m not wedded it to the transaction being stored in an item to be retrieved - but I don’t know where else I can store and retrieve.

Is there an approach to solve this?