code: func (f StringFlag) ApplyWithError(set *flag.FlagSet) error {
  	if envVal, ok := flagFromFileEnv(f.FilePath, f.EnvVar); ok {
  		f.Value = envVal
  	}
  
  	eachName(f.Name, func(name string) {
  		if f.Destination != nil {
  			set.StringVar(f.Destination, name, f.Value, f.Usage)
  			return
  		}
  		set.String(name, f.Value, f.Usage)
  	})
  
  	return nil
  }
docs: // ApplyWithError populates the flag given the flag set and environment