you could try these two items, have been changing quite a few on the newer cars. but its not a guarantee because a leak can be pratically anywhere.
1st try change both the cannister purge valve/solenoid and the vapor management valve.
2nd try - change the fuel tank cap
3. fuel pump gasket leaking?
another item to leak on a mustang is the filler neck tube goes through a grommet into the tank, that grommet is prone to leak.