If u already made the surface through thicken then just click on weldment command next to 3d sketch command in weldment tool bar and update cutlist and then expand it and do right click on given cutlist property then click properties there you link the weight with mass
If u already made the surface through thicken then just click on weldment command next to 3d sketch command in weldment tool bar and update cutlist and then expand it and do right click on given cutlist property then click properties there you link the weight with mass