function CustomMessage(const MsgName: String): String;
Returns the value of the [CustomMessages] entry with the specified name. If an entry with the specified name does not exist, an exception will be raised.
var
S: String;
begin
S := CustomMessage('CreateDesktopIcon');
// S = 'Create a &desktop icon'
S := FmtMessage(CustomMessage('UninstallProgram'), ['My Program']);
// S = 'Uninstall My Program'
end;