The temperature entered in this box should correspond to the temperature of the attachment in question. It would be reasonable to assume that vessel legs are much cooler than the actual metal temperature of the pressure vessel.
The controlling stress for leg and support lug design is the yield stress of the material at the leg/lug temperature. If the attachment is not at ambient, enter the yield stress at that temperature. This value is available in ASME Section II Part D.
Conversely, vessel lifting lugs use the basic allowable stress of the material for their design. CodeCalc will use the allowable stress of the metal at the design temperature.