You can observe how to add new Localization or change current messages in the resource above. However, if you are able to evaluate the needed message value, using the demonstrated approach (@Localizer["MyLocalizedOptionLabel"].Value), you should be able to set it as OptionalLabel
What is displayed is "MyLocalizedOptionLabel" instead of the value of the MyLocalizedOptionLabel key in the resource file. The same is true for OptionLabel in a popup editor.