That is a fairly unusual size for this purpose; probably your best bet would be to ask the trailer manufacturer, or alternatively consult a tyre specialist.
The tyres themselves will have a maximum pressure marked on the tyre wall; but that is indeed a maximum, and I would expect that for this relatively modest load you should use rather less pressure.
If all else fails, as a very tentative suggestion you could start with the stated maximum pressure and the corresponding load limit (also marked on the tyre wall), then adjust pro rata for the actual (laden) load. But if you can get advice from either the trailer manufacturer or a tyre specialist then so much the better.
Sorry that I can’t be of more help.
Oliver