What is the proper way to add .json files to WPF project?

I have a WPF project that gets data from a .json file. It works fine, however the WPF designer shows XDG0062 error and do not show data from file. I think it's heppening because program simply do not know if file exist or not. So I probably need to somehow add it to project, but is there a way to do it? And how properly do it if it's possible?

File contains a list of weights for map, list of objects on the map, and a background image. So the WPF designer does not show image. This how the image binded:

<Image.Source>
    <Binding Path="B64">
        <Binding.Converter>
            <c:Base64ToImage/>
        </Binding.Converter>
    </Binding>
</Image.Source>

I read the file by passing filename/path to constructor while creating object in ViewModel:

private Map map = new Map(filepath)