Connect a wire to a circuit board
If it's earth you want, you may be able to use a mounting hole. Show the mounting hole(s) on the schematic like any other component with a footprint and connect them where they should go (if anywhere). 

If it's a wire to be soldered into a hole into the board, again, make a component footprint (preferably with strain relief). 

You can also crimp a terminal onto the wire and solder the terminal into the board, use a board-to-wire connector pair (the 0.156" pitch series is good for high currents, whereas 2.54/2.50/2.0 mm pitch is good for finer individual wires. Or a screw terminal strip if connections are to be made in the field (there are even versions that use springs suitable for some applications that require no tools). Another possibility is to use Faston (tm) spades soldered into the board and female spade connectors crimped to the ends of the wires. The latter is very popular in appliance and commercial electronics that has to handle mains voltage at relatively high current. 



Again, find or make a component symbol and a footprint that suits the part you want to use.