code: func getFieldWithTag(obj interface{}, tagName string) *reflect.Value {
  	objVal := reflect.ValueOf(obj)
  	if objVal.Kind() != reflect.Struct {
  		objVal = objVal.Elem()
  	}
  
  	for i := 0; i < objVal.NumField(); i++ {
  		fieldType := objVal.Type().Field(i)
  		if tagName == fieldType.Tag.Get("jwt") {
  			field := objVal.Field(i)
  			return &field
  		}
  	}
  
  	return nil
  }
docs: // getFieldWithTag lookups jwt tag, with specified tagName on obj, returning
  // its reflected value.