An office environment during Winter can be a very dry one. Had good results blocking tops with "plastic wrap" used to wrap food items with. Just cover whole screen area that is not below lamp and affix with small pieces of tape to screen top edges. Normally, find it easier to cover whole top and then do a cut-out for lamp.

If that still does not do it; you might need to adapt a nozzle type humidifier to your cage or get one of those made for vivariums. Using higher wattage lamp will raise habitat temperature accordingly. Need to watch that too and avoid overheating frog.