First import the image, and enter lots of points along the top edge.
Select a group of points that you think could be modelled by a polynomial and use the “best Fit” option. The default ‘sig’ fig’ is 4, but a polynomial equation needs more accuracy to plot accurately. Use “PAGE – SETTINGS” to set the sig fig to 8. With the current best fit selected, use the PAGE menu and the option “Copy Status Bar”, and in a 3D page use “Enter equation” and paste this in, with “Plot as 2D” ticked, and Startup Options set to the domain.