Currently, Hermes have confirmed that on their side, for the majority of international countries, it's not possible to have the references appear on the shipping label. Shiptheory isn't able to directly edit the carrier label itself so this isn't something that can be edited.
However, using integrated labels with Shiptheory, it's possible to have the references appear on a slightly longer label. To do this, firstly navigate to your Integrated Labels setup here
As you can see by this example, you can set up your integrated labels to be slightly longer than the standard carrier labels so long as you have the paper to support this:
From there, you can then add the carrier label to the template, and the references just below:
This will then mean your labels can print with the needed information on the bottom without impacting the actual Hermes label.
If you have any issues from there, please Contact Support.