If it is anything like the Brompton branded tire they recommend:

Front: 75 to 90 PSI, 100 PSI max.
Rear: 85 to 100 PSI, 100 PSI max.

Schwalbe Marathon:
Front: 60 to 75 PSI, 115 PSI max.
Rear: 70 to 85 PSI, 115 PSI max.

I suspect closer to the first tire I listed.